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: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>