drm/amdgpu: add missing radeon secondary PCI ID

0x5b70 is a missing RV370 secondary id.  Add it so
we don't try and probe it with amdgpu.

Cc: michel@daenzer.net
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Tested-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org