Add the AP implementation for the Greybus Control Interface Suspend
Prepare Operation. AP uses this Operation during the Suspend transition
to request the Interface to enter a low-power mode 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>