usb: gadget: Add network gadget driver for dun and rmnet

This gadget driver works as independent data and control pass
through between modem device connected over USB or HSIC
and host/laptop.This driver communicates with modem device
using network bridge host driver.

Change-Id: Iaffdeaa6b5dbd601b73cba721a8f062d69912908
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>