litedram_gen: Add block_until_ready port parameter to control blocking behaviour.

In some cases, blocking the port until controller is ready is not wanted (ex on No-CPU
config where a port is used for the memtest).

examples/arty.yml

@@ -39,6 +39,7 @@
         },
         "wishbone_0" : {
             "type":  "wishbone",
+            "block_until_ready": True,
         },
         "native_0" : {
             "type": "native",

litedram/gen.py

@@ -654,13 +654,21 @@ class LiteDRAMCore(SoCCore):
             self.comb += wb_bus.connect_to_pads(wb_pads, mode="slave")
 
         # User ports -------------------------------------------------------------------------------
-        user_enable = Signal()
-        self.sync += user_enable.eq(self.ddrctrl.init_done.storage & ~self.ddrctrl.init_error.storage)
         self.comb += [
             platform.request("user_clk").eq(ClockSignal()),
             platform.request("user_rst").eq(ResetSignal()),
         ]
         for name, port in core_config["user_ports"].items():
+
+            # Common -------------------------------------------------------------------------------
+            user_enable = Signal()
+            # By default, block port until controller is ready.
+            if port.get("block_until_ready", True):
+                self.sync += user_enable.eq(self.ddrctrl.init_done.storage & ~self.ddrctrl.init_error.storage)
+            # Else never block.
+            else:
+                self.comb += user_enable.eq(1)
+
             # Native -------------------------------------------------------------------------------
             if port["type"] == "native":
                 user_port = self.sdram.crossbar.get_port(data_width=port.get("data_width", None))