dt-bindings: net: document new usxgmii phy mode

Add new interface mode USXGMII to binding documentation.

Signed-off-by: Heiner Kallweit <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/ethernet.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