usb: gadget: FunctionFS: create utility file

A header file to be used by f_fs.c and g_ffs.c will be required when
f_fs.c is converted into a module.

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>