drm/amdgpu: Include sienna_cichlid in USBC PD FW support.

Create sysfs interface also for sienna_cichlid.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>