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>

Documentation/devicetree/bindings/usb/usb251xb.txt

@@ -64,8 +64,10 @@ Optional properties :
  - power-on-time-ms : Specifies the time it takes from the time the host
 	initiates the power-on sequence to a port until the port has adequate
 	power. The value is given in ms in a 0 - 510 range (default is 100ms).
- - swap-dx-lanes : Specifies the ports which will swap the differential-pair
-	(D+/D-), default is not-swapped.
+ - swap-dx-lanes : Specifies the downstream ports which will swap the
+	differential-pair (D+/D-), default is not-swapped.
+ - swap-us-lanes : Selects the upstream port differential-pair (D+/D-)
+	swapping (boolean, default is not-swapped)
 
 Examples:
 	usb2512b@2c {