drm/amd/pm: enable Sienna Cichlid overdrive support

Enable Sienna Cichlid gfxclk/uclk overdrive support.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>