greybus: gpio: Add runtime_pm suppourt

greybus: gpio: Add runtime_pm suppourt

Add runtime pm support for the gpio driver.
Since there is no remote wakeup support, the
module will not suspend as long as a gpio is
requested.

Maybe an optimization could be made, to allow suspend
if all the requested gpios are in output mode, since
the bridge should maintain the state of the gpio
during suspend.

Testing Done: using the test board, let the
gpbrige enter standby and request a gpio.

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>