drm/amdgpu: add new amdgpu nbio header file

More nbio funcitonalities will be added and nbio could
be treated as an ip block like gfx/sdma.etc

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>