usb: gadget: Add snapshot of USB CDEV function driver

This change adds USB CDEV function driver which creates USB
character device to provide DUN and NMEA functionality.
This snapshot is taken as of commit 4f7918fd623f ("usb: f_gsi: Use
required RNDIS IAD descriptors with full speed descriptors").

Replace usage of idr_is_empty() with ida_is_empty() in f_cdev.c
as ida no longer uses an internal idr.

Change-Id: Icfdc5c3b8e472d296a393ebb0be36eb5379c3b21
Signed-off-by: Jack Pham <jackp@codeaurora.org>