drm/amdgpu: Add memory partitions to gmc

Some ASICs have the device memory divided into multiple partitions. The
parititions could be denoted by a numa node or by a range of pages.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>