usb: gadget: Add the console support for usb-to-serial port

It dose not work when we want to use the usb-to-serial port based
on one usb gadget as a console. Thus this patch adds the console
initialization to support this request.

To avoid the re-entrance when transferring data with usb endpoint,
it introduces a kthread to do the IO transmission.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>