usb: gadget: FunctionFS: add devices management code

This will be required in order to use the new function interface
(usb_get_function_instance/usb_put_function_instance)

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyunmgin Park <kyungmin.park@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>