usb: gadget: f_fs: OS descriptors support

Add support for OS descriptors. The new format of descriptors is used,
because the "flags" field is required for extensions. os_count gives
the number of OSDesc[] elements.
The format of descriptors is given in include/uapi/linux/usb/functionfs.h.

For extended properties descriptor the usb_ext_prop_desc structure covers
only a part of a descriptor, because the wPropertyNameLength is unknown
up front.

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