greybus: uart: Add runtime pm support

This adds runtime pm support for the uart driver.

Testing Done: Using the test daughter board, let
the gpb bridge enter standby and do a uart transfer
operation.

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