drm/amdgpu/swsmu: add SMU mailbox registers in SMU context

So we can eventaully use them in the common smu code for
accessing the SMU mailboxes without needing a lot of
per asic logic in the common code.

Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>