drm/amdgpu: add XGMI support for sienna cichlid

support for setting up XGMI FB address regions

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>