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 <2e7ac4ed668> ("Merge
"msm: ipa: Fix to add string NULL terminator"").

Change-Id: I2a20aec1c21a506f8981e907bc0f0f58fff7fe58
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>