greybus: i2c: add runtime pm support

Add runtime pm support to camera i2c bridged phy device class driver

Testing Done:
 - Passed #gb_test.sh -v -t i2c

Signed-off-by: David Lin <dtwlin@google.com>
Signed-off-by: Axel Haslam <haslam_axel@projectara.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Alex Elder <elder@linaro.org>