drm/amdgpu/gmc: add initial xgmi structure to amdgpu_gmc structure

Initial pass at a structure to store xgmi info.  xgmi is a high
speed cross gpu interconnect.

Acked-by: Huang Rui <ray.huang@amd.com>
Acked-by: Slava Abramov <slava.abramov@amd.com>
Reviewed-by :Shaoyun liu <Shaoyun.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Shaoyun Liu <Shaoyun.Liu@amd.com>