usb: gadget: configfs: OS String support

Add handling of OS String extension from the configfs interface.
A directory "os_desc" is added at the top level of a gadget's
directories hierarchy. In the "os_desc" directory there are
three attributes: "use", "b_vendor_code" and "qw_sign".
If "use" contains "0" the OS string is not reported to the host.
"b_vendor_code" contains a one-byte value which is used
for custom per-device and per-interface requests.
"qw_sign" contains an identifier to be reported as the "OS String"
proper.

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