target/arm: Implement v8.1M REVIDR register

In v8.1M a REVIDR register is defined, which is at address 0xe00ecfc
and is a read-only IMPDEF register providing implementation specific
minor revision information, like the v8A REVIDR_EL1. Implement this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201119215617.29887-19-peter.maydell@linaro.org

hw/intc/armv7m_nvic.c

@@ -1025,6 +1025,11 @@ static uint32_t nvic_readl(NVICState *s, uint32_t offset, MemTxAttrs attrs)
         }
         return val;
     }
+    case 0xcfc:
+        if (!arm_feature(&cpu->env, ARM_FEATURE_V8_1M)) {
+            goto bad_offset;
+        }
+        return cpu->revidr;
     case 0xd00: /* CPUID Base.  */
         return cpu->midr;
     case 0xd04: /* Interrupt Control State (ICSR) */