drm/amdgpu: add GMC 7.1 register headers

These are register headers for the GMC (Graphics Memory Controller)
block on the GPU.

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