greybus: camera: add runtime pm support

Add runtime pm support to camera protocol device class driver.

Testing Done:
 - Make sure white camera module is able to runtime suspend and resume
   when the camera is being used

Signed-off-by: David Lin <dtwlin@google.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@linaro.org>
Signed-off-by: Alex Elder <elder@linaro.org>