In commit 01765386 we fixed a bug where we weren't correctly
bracketing changes to some registers with pmu_op_start() and
pmu_op_finish() calls for changes which affect whether the PMU
counters might be enabled.  However, we missed the case of writes to
the AArch64 MDCR_EL3 register, because (unlike its AArch32
counterpart) they are currently done directly to the CPU state struct
without going through the sdcr_write() function.

Give MDCR_EL3 a writefn which handles the PMU start/finish calls.
The SDCR writefn then simplfies to "call the MDCR_EL3 writefn after
masking off the bits which don't exist in the AArch32 register".

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220923123412.1214041-3-peter.maydell@linaro.org