greybus: control: add interface deactivate prepare operation

Add the AP implementation for the Greybus Control Interface Deactivate
Prepare Operation. AP uses this Operation during the Power Down
transition to request the bridge to power down after it detects a
subsequent UniPro link hibernation.

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