drm/amdgpu/discovery: Add gmc v12_0 ip block

Add gmc v12_0 ip block.

v2: Squash in updates (Alex)

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>