tools: Add initial LiteX standalone SoC generator.

Allow generating standalone SoC with CPU/Peripherals that can be re-integrated in
external design or top level LiteX SoCs.

Example of of use:
python3 litex_soc_gen.py --cpu-type=vexriscv --bus-standard=wishbone --build
python3 litex_soc_gen.py --cpu-type=naxriscv --bus-standard=axi-lite --build

litex/tools/litex_soc_gen.py

@@ -0,0 +1,126 @@
+#!/usr/bin/env python3
+
+#
+# This file is part of LiteX.
+#
+# Copyright (c) 2022 Florent Kermarrec <florent@enjoy-digital.fr>
+# SPDX-License-Identifier: BSD-2-Clause
+
+"""
+LiteX standalone SoC generator.
+
+This generator reduces the scope of LiteX to CPU/Peripherals selection/integration and to the creation
+of SoC with MMAP/Streaming DMA interfaces that can be reintegrated in external designs (or LiteX SoC).
+Think of it as a mini Nios SOPC Builder/ Zynq or Microblaze Subsystem generator that offers you the
+possibility to reuse any of CPU supported by LiteX :)
+"""
+
+import argparse
+
+from migen import *
+
+from litex.build.generic_platform import *
+
+from litex.soc.integration.soc_core import *
+from litex.soc.integration.soc import SoCRegion
+from litex.soc.integration.builder import *
+from litex.soc.interconnect import wishbone
+from litex.soc.interconnect import axi
+
+# IOs/Interfaces -----------------------------------------------------------------------------------
+
+def get_common_ios():
+    return [
+        # Clk/Rst.
+        ("clk", 0, Pins(1)),
+        ("rst", 0, Pins(1)),
+    ]
+
+def get_uart_ios():
+    return [
+        # Serial
+        ("uart", 0,
+            Subsignal("tx",  Pins(1)),
+            Subsignal("rx", Pins(1)),
+        )
+    ]
+
+# Platform -----------------------------------------------------------------------------------------
+
+class Platform(GenericPlatform):
+    def build(self, fragment, build_dir, build_name, **kwargs):
+        os.makedirs(build_dir, exist_ok=True)
+        os.chdir(build_dir)
+        conv_output = self.get_verilog(fragment, name=build_name)
+        conv_output.write(f"{build_name}.v")
+
+# LiteX SoC Generator ------------------------------------------------------------------------------
+
+class LiteXSoCGenerator(SoCMini):
+    def __init__(self, name="litex_soc", sys_clk_freq=int(50e6), **kwargs):
+        # Platform ---------------------------------------------------------------------------------
+        platform = Platform(device="", io=get_common_ios())
+        platform.name = name
+        platform.add_extension(get_uart_ios())
+
+        # CRG --------------------------------------------------------------------------------------
+        self.submodules.crg = CRG(
+            clk = platform.request("clk"),
+            rst = platform.request("rst"),
+        )
+
+        # SoC --------------------------------------------------------------------------------------
+        if kwargs["uart_name"] == "serial":
+            kwargs["uart_name"] = "uart"
+        SoCCore.__init__(self, platform, clk_freq=sys_clk_freq, **kwargs)
+
+        # MMAP Slave Interface ---------------------------------------------------------------------
+        s_bus = {
+            "wishbone" : wishbone.Interface(),
+            "axi-lite" : axi.AXILiteInterface(),
+
+        }[kwargs["bus_standard"]]
+        self.bus.add_master(name="mmap_s", master=s_bus)
+        platform.add_extension(s_bus.get_ios("mmap_s"))
+        wb_pads = platform.request("mmap_s")
+        self.comb += s_bus.connect_to_pads(wb_pads, mode="slave")
+
+        # MMAP Master Interface --------------------------------------------------------------------
+        # FIXME: Allow Region configuration.
+        m_bus = {
+            "wishbone" : wishbone.Interface(),
+            "axi-lite" : axi.AXILiteInterface(),
+
+        }[kwargs["bus_standard"]]
+        wb_region = SoCRegion(origin=0x2000_0000, size=0x1000_0000, cached=True) # FIXME.
+        self.bus.add_slave(name="mmap_m", slave=m_bus, region=wb_region)
+        platform.add_extension(m_bus.get_ios("mmap_m"))
+        wb_pads = platform.request("mmap_m")
+        self.comb += m_bus.connect_to_pads(wb_pads, mode="master")
+
+# Build --------------------------------------------------------------------------------------------
+def main():
+    # Arguments.
+    from litex.soc.integration.soc import LiteXSoCArgumentParser
+    parser = LiteXSoCArgumentParser(description="LiteX standalone SoC generator")
+    target_group = parser.add_argument_group(title="Generator options")
+    target_group.add_argument("--name",          default="litex_soc", help="SoC Name.")
+    target_group.add_argument("--build",         action="store_true", help="Build SoC.")
+    target_group.add_argument("--sys-clk-freq",  default=int(50e6),   help="System clock frequency.")
+    builder_args(parser)
+    soc_core_args(parser)
+    args = parser.parse_args()
+
+    # SoC.
+    soc = LiteXSoCGenerator(
+        name         = args.name,
+        sys_clk_freq = int(float(args.sys_clk_freq)),
+        **soc_core_argdict(args)
+    )
+
+    # Build.
+    builder = Builder(soc, **builder_argdict(args))
+    builder.build(build_name=args.name, run=args.build)
+
+if __name__ == "__main__":
+    main()