dt-bindings: net: document new usxgmii phy mode

Add new interface mode USXGMII to binding documentation.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ethernet.txt

@@ -43,6 +43,7 @@ Documentation/devicetree/bindings/phy/phy-bindings.txt.
   * "rxaui"
   * "xaui"
   * "10gbase-kr" (10GBASE-KR, XFI, SFI)
+  * "usxgmii"
 - phy-connection-type: the same as "phy-mode" property but described in the
   Devicetree Specification;
 - phy-handle: phandle, specifies a reference to a node representing a PHY