usb: usb251xb: Document chip variant usb2422

The next patch introduces support for the USB2422. Add it to the list of
devices supported by the binding.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191023105250.16537-2-u.kleine-koenig@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>