usb: gadget: Add support for usb runtime pm APIs

Add usb_gadget_autompm_get* and usb_gadget_autompm_put*
APIs. These APIs are used by function drivers 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: I17a48bd0d313f94e5079e31ed69bb916025c594f
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>