usb: gadget: f_gsi: Support power management using usb auto PM APIs

PM runtime APIs are used by function driver to synchronize between
controller low power mode and function driver specific tasks which
require controller to be active in order to finish them.

Change-Id: Ic9552d89c7eaeec53847364f05006ca72eb92f3a
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>