drm/amdgpu/sienna_cichlid: add SMU i2c support (v2)

Enable SMU i2c bus access for sienna_cichlid asics.

v2: change callback name

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