usb: f_serial: Add function suspend/resume for super-speed wakeup

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.

CRs-Fixed: 964728
Change-Id: I6bb87b334ca1a5363c9d8e607c7a3d07deab04f2
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>