usb: usb251xb: Add US lanes inversion dts-bindings

Since a separate US ports lanes polarity inversion property is going to
be available the bindings doc-file should be updated with information
about swap-us-lanes bool property, which will be responsible for it.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Richard Leitner <richard.leitner@skidata.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>