greybus: audio: add runtime pm support

Add runtime pm support to audio protocol device class driver.

Testing Done:
 - Use white speaker module and check the interface is autosuspended when
   it's idle and resumed when playback audio

Signed-off-by: David Lin <dtwlin@google.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Reviewed-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Alex Elder <elder@linaro.org>