drm/amdgpu: add sdma ip block for sienna_cichlid (v5)

Sienna_Cichlid have 4 sdma controllers.

v2: add missing license to sdma_common.h (Alex)
v3: rebase (Alex)
v4: squash in policy fix (Alex)
v4: squash in fw_name fix

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>