target/arm: Enable FEAT_DGH for -cpu max

This extension concerns not merging memory access, which TCG does
not implement.  Thus we can trivially enable this feature.
Add a comment to handle_hint for the DGH instruction, but no code.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220506180242.216785-23-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>