target-microblaze: mmu: Make the TLBX MISS bit read-only

Make the TLBX MISS bit read-only.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>

target/microblaze/mmu.c

@@ -273,6 +273,10 @@ void mmu_write(CPUMBState *env, uint32_t rn, uint32_t v)
                 env->mmu.regs[rn] = v;
             }
             break;
+        case MMU_R_TLBX:
+            /* Bit 31 is read-only.  */
+            env->mmu.regs[rn] = deposit32(env->mmu.regs[rn], 0, 31, v);
+            break;
         case MMU_R_TLBSX:
         {
             struct microblaze_mmu_lookup lu;