It doesn't expose PPTable descriptor on APU platform. So max/min
temperature values cannot be got from APU platform.

v2: Stoney needs to skip crit temperature as well.
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>