target/mips/op_helper: Document Invalidate/Writeback opcodes as no-op

QEMU does not model caches, so there is not much to do with the
Invalidate/Writeback opcodes. Make it explicit adding a comment.

Suggested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20200813181527.22551-3-f4bug@amsat.org>

target/mips/op_helper.c

@@ -1586,6 +1586,11 @@ void helper_cache(CPUMIPSState *env, target_ulong addr, uint32_t op)
         memory_region_dispatch_read(env->itc_tag, index, &env->CP0_TagLo,
                                     MO_64, MEMTXATTRS_UNSPECIFIED);
         break;
+    case 0b000: /* Index Invalidate */
+    case 0b100: /* Hit Invalidate */
+    case 0b110: /* Hit Writeback */
+        /* no-op */
+        break;
     default:
         break;
     }