usb: gadget: Fix remote wakeup during low-power mode

Fix remote wakeup failure during low-power mode. The remote wakeup
operation issues an asynchronous low-power mode exit in a remote task
and then immediately continues to access the USB hardware for priming
the endpoint. In some cases it might happen that the remote task that
performs the low-power mode exit hasn't completed before the endpoint
priming and therefore a failure occurs. The fix adds a 'remote wakeup'
queue which holds the requests until the USB HW exits from low-power mode
and resumes.After resume, the pending requests are pushed to the hardware.

CRs-fixed: 470164
Change-Id: I3e572a082d0e93560bf69889e269fa34336cf5e8
Signed-off-by: Daniel Segal <dsegal@codeaurora.org>