drm/amd/powerplay: add enum smu_msg_type to header

each asic with different message index,
add this header to help top level smu code to send message.

Signed-off-by: Kevin Wang <Kevin1.Wang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>