target/arm: Update define_one_arm_cp_reg_with_opaque for VHE

For ARMv8.1, op1 == 5 is reserved for EL2 aliases of
EL1 and EL0 registers.

Backports commit b4ecf60f7eee88cbfe5700044790cb7494c5dd37 from qemu

qemu/target/arm/helper.c

@@ -7324,13 +7324,10 @@ void define_one_arm_cp_reg_with_opaque(ARMCPU *cpu,
             mask = PL0_RW;
             break;
         case 4:
+        case 5:
             /* min_EL EL2 */
             mask = PL2_RW;
             break;
-        case 5:
-            /* unallocated encoding, so not possible */
-            assert(false);
-            break;
         case 6:
             /* min_EL EL3 */
             mask = PL3_RW;