drm/amdgpu: added a sysfs interface for thermal throttling

added a sysfs interface for thermal throttling, then userspace
can get/update thermal limit

Signed-off-by: Kun Liu <Kun.Liu2@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>