drm/amdgpu: Add documentation for memory info

Add the amdgpu.rst tie-ins for the mem_info documentation

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>