USB: gadget: Add debugfs node to initiate function remote wakeup

In USBCV Function remote wakeup enable test requires to initiate
function wakeup notification from the device. Hence add a debugfs
node to initiate function wakeup notification from the device.
Use below commands to initiate function wakeup notification
for Modem:
echo > /sys/kernel/debug/usb_gbridge/remote_wakeup

and for NMEA:
echo > /sys/kernel/debug/usb_serial0/remote_wakeup

Change-Id: Icf13454355d1ce6eaeb4720cbb4134adfee194ae
CRs-Fixed: 1017394
Signed-off-by: Arumuga Durai A <cadurai@codeaurora.org>