greybus: interface: implement unipro link hibernate call

Adds AP implementation of unipro link hibernation set power mode call
needed for proper cport closure and interface suspend and power off
transition.

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