greybus: loopback: add runtime pm support

Add runtime pm to the loopback driver so that
the module wakes up from suspend while a test
is executed.

Testing Done: Let the module enter standby and
execute a loopback test.

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>