drm/amdgpu: add gmc cg support for sienna_cichlid

Add gmc clockgating support for sienna_cichlid.
The athub version used for sienna_cichlid is v2.1.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>