target-arm: Add MDCR_EL2

target-arm: Add MDCR_EL2

Add the MDCR_EL2 register. We don't implement any of
the debug-related traps this register controls yet, so
currently it simply reads back as written.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1444383794-16767-1-git-send-email-serge.fdrv@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: tweaked commit message; moved non-dummy definition from
debug_cp_reginfo to el2_cp_reginfo.]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>