usb: gadget: Add link power management support

Link Power Management (a.k.a. L1) is similar to the existing usb bus
suspend/resume/remote-wakeup, but has transitional latencies of tens
of microseconds between power states (instead of three to greater than
20 millisecond latencies of the USB 2.0 suspend/resume).

Change-Id: I8ae493534702e658c24f384a6b705b08e9ea9d05
Signed-off-by: Shimrit Malichi <smalichi@codeaurora.org>