usb: gadget: f_obex: add configfs support

f_obex learns about our new configfs-based interface,
which will allow gadgets to be bound to controllers
through userland.

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>