target/arm: Honour HCR_EL2.TGE and MDCR_EL2.TDE in debug register access checks

target/arm: Honour HCR_EL2.TGE and MDCR_EL2.TDE in debug register access checks

Some debug registers can be trapped via MDCR_EL2 bits TDRA, TDOSA,
and TDA, which we implement in the functions access_tdra(),
access_tdosa() and access_tda(). If MDCR_EL2.TDE or HCR_EL2.TGE
are 1, the TDRA, TDOSA and TDA bits should behave as if they were 1.
Implement this by having the access functions check MDCR_EL2.TDE
and HCR_EL2.TGE.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180724115950.17316-3-peter.maydell@linaro.org