linux-phytec-fsl

usb: gadget: f_serial: add configfs support

this patch implements the new configfs based interface
on f_serial function driver.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>