usb: gadget: f_serial: Add support for DUN

Currently driver only support generic serial interface.
Adding communication device class specific descriptors
and notification endpoint support required for the
enumeration of a DUN interface. Add control transfer
APIs to send and receive serial line state.

Change-Id: I8d50495ad51797dfc8b3e77464474d9fe70d8636
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>