arch-riscv: added dummy implementation of wfi instruction.

Change-Id: I7ca86f57c7d794bf544e34e747875049a3f29d6a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25653
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/arch/riscv/isa/decoder.isa

@@ -1794,6 +1794,9 @@ decode QUADRANT default Unknown::unknown() {
                                 NPC = xc->readMiscReg(MISCREG_SEPC);
                             }
                         }}, IsReturn);
+                        0x5: wfi({{
+                            // don't do anything for now
+                        }}, No_OpClass);
                     }
                     0x9: sfence_vma({{
                         xc->tcBase()->getITBPtr()->demapPage(Rs1, Rs2);