usb: gadget: f_loopback: add configfs support

Add support for using the loopback USB function in gadgets composed with
configfs.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/Documentation/ABI/testing/configfs-usb-gadget-loopback b/Documentation/ABI/testing/configfs-usb-gadget-loopback
new file mode 100644
index 0000000..852b236
--- /dev/null
+++ b/Documentation/ABI/testing/configfs-usb-gadget-loopback
@@ -0,0 +1,8 @@
+What:		/config/usb-gadget/gadget/functions/Loopback.name
+Date:		Nov 2013
+KenelVersion:	3.13
+Description:
+		The attributes:
+
+		qlen		- depth of loopback queue
+		bulk_buflen	- buffer length