drm/amd/powerplay: add smu12_driver_if.h (v3)

This patch adds smu12_driver_if.h

v2: squash in updates (Alex)
v3: more updates (Alex)

Signed-off-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>