dt-bindings: usb: usb251xb: add documentation for data lane swapping

Add optional binding to allow USB differential-pair (D+/D-) data lane
swapping. The swapping can be specified for each port separately,
default is no swapping.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Richard Leitner <richard.leitner@skidata.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>