target-arm: Implement AArch64 dummy MDSCR_EL1

We don't support letting the guest do debug, but Linux prods the
monitor debug system control register anyway, so implement a dummy
RAZ/WI version.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>

target-arm/helper.c

@@ -1656,6 +1656,12 @@ static const ARMCPRegInfo v8_cp_reginfo[] = {
       .opc0 = 1, .opc2 = 0, .crn = 8, .crm = 7, .opc2 = 7,
       .access = PL1_W, .type = ARM_CP_NO_MIGRATE,
       .writefn = tlbi_aa64_vaa_write },
+    /* Dummy implementation of monitor debug system control register:
+     * we don't support debug.
+     */
+    { .name = "MDSCR_EL1", .state = ARM_CP_STATE_AA64,
+      .opc0 = 2, .opc1 = 0, .crn = 0, .crm = 2, .opc2 = 2,
+      .access = PL1_RW, .type = ARM_CP_CONST, .resetvalue = 0 },
     REGINFO_SENTINEL
 };