usb: gadget: f_eem: add configfs support

f_eem learns about our configfs interface so we
can remove in-kernel gadget drivers in future.

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>