greybus: interface: add runtime pm support

Configure and enable runtime pm support for the Interface. Refer to the
12.2. The Interface Lifecycle of the Greybus specification for details
on the requirements for transitioning from ENUMERATED to SUSPEND and
vice versa. All the Bundles for the Interface have to be either OFF or
SUSPENDED before the Interface can be autosuspended.

Testing Done:
 - Check the runtime_status of an interface driver and validate the
   suspend current of a module.

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>