soc/SoCController: Add separate fields for SoC and CPU resets.

As discussed in #909, in some specific cases, it can be interesting to be able
to keep the CPU in reset while the rest of the SoC is still operating (ex the
peripherals/bridges).

With theses changes, the old behaviour is preserved to do a full SoC Reset (at
the exception that writing a 1 is now mandatory) and a separate field specific
to the CPU reset is added.

The SoC Reset is a pulse (otherwise the system would be stuck in Reset) while
the CPU Reset is based on the register value (so can be pulse or hold).

CHANGES

@@ -8,6 +8,7 @@
 	[> Added Features
 	-----------------
 	- cpu/vexriscv:   Add CFU support.
+	- soc/controller: Add separate SoC/CPU reset fields.
 
 	[> API changes/Deprecation
 	--------------------------

litex/soc/integration/soc.py

@@ -658,7 +658,10 @@ class SoCIRQHandler(SoCLocHandler):
 class SoCController(Module, AutoCSR):
     def __init__(self, with_reset=True, with_scratch=True, with_errors=True):
         if with_reset:
-            self._reset = CSRStorage(1, description="""Any write to this register will reset the SoC.""")
+            self._reset = CSRStorage(fields=[
+                CSRField("soc_rst", size=1, offset=0, pulse=True, description="""Write `1` to this register to reset the full SoC (Pulse Reset)"""),
+                CSRField("cpu_rst", size=1, offset=1,             description="""Write `1` to this register to reset the CPU(s) of the SoC (Hold Reset)"""),
+            ])
         if with_scratch:
             self._scratch = CSRStorage(32, reset=0x12345678, description="""
                 Use this register as a scratch space to verify that software read/write accesses
@@ -671,8 +674,8 @@ class SoCController(Module, AutoCSR):
 
         # Reset
         if with_reset:
-            self.reset = Signal()
+            self.soc_rst = self._reset.fields.soc_rst
-            self.comb += self.reset.eq(self._reset.re)
+            self.cpu_rst = self._reset.fields.cpu_rst
 
         # Errors
         if with_errors:
@@ -918,8 +921,11 @@ class SoC(Module):
 
             # Connect SoCController's reset to CPU reset.
             if hasattr(self, "ctrl"):
-                if hasattr(self.ctrl, "reset"):
+                self.comb += self.cpu.reset.eq(
-                    self.comb += self.cpu.reset.eq(self.ctrl.reset)
+                    # Reset the CPU on...
+                    getattr(self.ctrl, "soc_rst", 0) | # Full SoC Reset command...
+                    getattr(self.ctrl, "cpu_rst", 0)   # or on CPU Reset command.
+                )
             self.add_config("CPU_RESET_ADDR", reset_address)
 
         # Add CPU's SoC components (if any).
@@ -952,11 +958,11 @@ class SoC(Module):
         }[self.bus.standard]
 
         # SoC Reset --------------------------------------------------------------------------------
-        # Connect SoCController's reset to CRG's reset if presents.
+        # Connect soc_rst to CRG's rst if presents.
         if hasattr(self, "ctrl") and hasattr(self, "crg"):
-            if hasattr(self.ctrl, "_reset") and hasattr(self.crg, "rst"):
+            crg_rst = getattr(self.crg, "rst", None)
-                if isinstance(self.crg.rst, Signal):
+            if isinstance(crg_rst, Signal):
-                    self.comb += self.crg.rst.eq(self.ctrl._reset.re)
+                self.comb += crg_rst.eq(getattr(self.ctrl, "soc_rst", 0))
 
         # SoC CSR bridge ---------------------------------------------------------------------------
         # FIXME: for now, use registered CSR bridge when SDRAM is present; find the best compromise.