target-arm: Implement MDCR_EL3.TDA and MDCR_EL2.TDA traps

Implement the debug register traps controlled by MDCR_EL2.TDA
and MDCR_EL3.TDA.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>