usb: gadget: FunctionFS: add configfs support

Add support for using FunctionFS in configfs-based USB gadgets.

[ balbi@ti.com : removed redefinition of VERBOSE_DEBUG and few
	trailing whitespaces ]

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