target-arm: Remove comment about MDSCR_EL1 being dummy implementation

MDSCR_EL1 has actual functionality now; remove the out of date
comment that claims it is a dummy implementation.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target-arm/helper.c

@@ -2244,9 +2244,7 @@ static const ARMCPRegInfo debug_cp_reginfo[] = {
       .access = PL1_R, .type = ARM_CP_CONST, .resetvalue = 0 },
     { .name = "DBGDSAR", .cp = 14, .crn = 2, .crm = 0, .opc1 = 0, .opc2 = 0,
       .access = PL0_R, .type = ARM_CP_CONST, .resetvalue = 0 },
-    /* Dummy implementation of monitor debug system control register:
-     * we don't support debug. (The 32-bit alias is DBGDSCRext.)
-     */
+    /* Monitor debug system control register; the 32-bit alias is DBGDSCRext. */
     { .name = "MDSCR_EL1", .state = ARM_CP_STATE_BOTH,
       .cp = 14, .opc0 = 2, .opc1 = 0, .crn = 0, .crm = 2, .opc2 = 2,
       .access = PL1_RW,