target-arm: Implement MDSCR_EL1 as having state

Now that all the new code to support single-stepping is in
place, wire up the guest-visible MDSCR_EL1, so the guest
can enable single-stepping.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NEdgar E. Iglesias <edgar.iglesias@xilinx.com>

target-arm/helper.c

@@ -2204,7 +2204,9 @@ static const ARMCPRegInfo debug_cp_reginfo[] = {
      */
     { .name = "MDSCR_EL1", .state = ARM_CP_STATE_BOTH,
       .cp = 14, .opc0 = 2, .opc1 = 0, .crn = 0, .crm = 2, .opc2 = 2,
-      .access = PL1_RW, .type = ARM_CP_CONST, .resetvalue = 0 },
+      .access = PL1_RW,
+      .fieldoffset = offsetof(CPUARMState, cp15.mdscr_el1),
+      .resetvalue = 0 },
     /* We define a dummy WI OSLAR_EL1, because Linux writes to it. */
     { .name = "OSLAR_EL1", .state = ARM_CP_STATE_BOTH,
       .cp = 14, .opc0 = 2, .opc1 = 0, .crn = 1, .crm = 0, .opc2 = 4,