subsys: usb: class: add loopback function

Add loopback function. This function can be used to test
USB device drivers and device stack connected to linux host
and has the similar interface as "Gadget Zero" [1] of the Linux
kernel.

Use modprobe usbtest to load the module, see also [2] for the
description of the tests and for Vendor and Product ID of the
"Gadget Zero". The userspace tool testusb [3] is needed to start
the tests.

[1] linux/drivers/usb/gadget/function/f_loopback.c
[2] linux/drivers/usb/misc/usbtest.c
[3] linux/tools/usb/testusb.c

Signed-off-by: Johann Fischer <j.fischer@phytec.de>