drm/amdgpu/soc15: add support for vega12

Add the IP blocks, clock and powergating flags, and
common clockgating support.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>