drm/amdgpu: add new asic_type for IP discovery

Add a new asic type for asics where we don't have an
explicit entry in the PCI ID list.  We don't need
an asic type for these asics, other than something higher
than the existing ones, so just use this for all new
asics.

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