greybus: gbphy: add gbphy runtime pm support

Since GBphy is a child of the Bundle device driver, for those runtime pm
settings that are common to all the protocol drivers need to go in to
the GBphy bus driver.

Testing Done:
 - Check gbphy driver can be autosuspended

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