usb: f_cdev: Add function suspend support for serial interfaces

USB super-speed mode requires device to advertise remote wakeup
capability at function driver level called as function
wakeup capability. Host queries this using get_status and
uses set_feature to enable function remote wakeup before
initiating function suspend. This is required to initiate
remote wakeup as well which is failed by udc otherwise in SS mode.

Also Add debugfs node to initiate function remote wakeup for serial
interfaces. 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 1 > /sys/kernel/debug/at_usb0/remote_wakeup

for NMEA:
echo 1 > /sys/kernel/debug/at_usb1/remote_wakeup.

Change-Id: I1c4c02282a50dc96dd67b646788a5e08818c34c0
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>