drm/amd/powerplay: add iceland SMU mananger

The system management unit (SMU) is a subcomponent of the northbridge
that is responsible for a variety of system and power management tasks
during boot and runtime for GPU. In powerplay, it will be used on
firmware loading and power task management. This patch adds SMU
mananger for iceland.

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