[ibex,fpga] Fix FPGA related Ibex RF issue

This commit manually pulls over lowRISC/ibex#2224 to fix
the register file for the FPGA configuration. As we do not
want to include other Ibex changes, this is done using a
patch file.

Signed-off-by: Pascal Nasahl <nasahlpa@lowrisc.org>
(commit is original to earlgrey_1.0.0)

hw/vendor/lowrisc_ibex/rtl/ibex_register_file_fpga.sv

@@ -147,14 +147,14 @@ module ibex_register_file_fpga #(
       .out_o (mem_o_b)
     );
 
-    assign rdata_a_o = (raddr_a_i == '0) ? '0 : mem_o_a;
-    assign rdata_b_o = (raddr_b_i == '0) ? '0 : mem_o_b;
+    assign rdata_a_o = (raddr_a_i == '0) ? WordZeroVal : mem_o_a;
+    assign rdata_b_o = (raddr_b_i == '0) ? WordZeroVal : mem_o_b;
   end else begin : gen_no_rdata_mux_check
     // async_read a
-    assign rdata_a_o = (raddr_a_i == '0) ? '0 : mem[raddr_a_i];
+    assign rdata_a_o = (raddr_a_i == '0) ? WordZeroVal : mem[raddr_a_i];
 
     // async_read b
-    assign rdata_b_o = (raddr_b_i == '0) ? '0 : mem[raddr_b_i];
+    assign rdata_b_o = (raddr_b_i == '0) ? WordZeroVal : mem[raddr_b_i];
   end
 
   // we select

hw/vendor/patches/lowrisc_ibex/rtl/0001-Fix-fpga-register-file.patch

@@ -0,0 +1,23 @@
+diff --git a/ibex_register_file_fpga.sv b/ibex_register_file_fpga.sv
+index 7a0ae34b15..73e0afab2f 100644
+--- a/ibex_register_file_fpga.sv
++++ b/ibex_register_file_fpga.sv
+@@ -147,14 +147,14 @@ module ibex_register_file_fpga #(
+       .out_o (mem_o_b)
+     );
+
+-    assign rdata_a_o = (raddr_a_i == '0) ? '0 : mem_o_a;
+-    assign rdata_b_o = (raddr_b_i == '0) ? '0 : mem_o_b;
++    assign rdata_a_o = (raddr_a_i == '0) ? WordZeroVal : mem_o_a;
++    assign rdata_b_o = (raddr_b_i == '0) ? WordZeroVal : mem_o_b;
+   end else begin : gen_no_rdata_mux_check
+     // async_read a
+-    assign rdata_a_o = (raddr_a_i == '0) ? '0 : mem[raddr_a_i];
++    assign rdata_a_o = (raddr_a_i == '0) ? WordZeroVal : mem[raddr_a_i];
+
+     // async_read b
+-    assign rdata_b_o = (raddr_b_i == '0) ? '0 : mem[raddr_b_i];
++    assign rdata_b_o = (raddr_b_i == '0) ? WordZeroVal : mem[raddr_b_i];
+   end
+
+   // we select