Documentation: devicetree: update dsa binding with optional properties

Add documentation for a bunch of new optional properties described in
ethernet.txt and fixed-link.txt, this includes: 'phy-handle', 'phy-mode'
and the 'fixed-link' subnode.
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/dsa.txt

@@ -39,6 +39,22 @@ Optionnal property:
 			  This property is only used when switches are being
 			  chained/cascaded together.
 
+- phy-handle		: Phandle to a PHY on an external MDIO bus, not the
+			  switch internal one. See
+			  Documentation/devicetree/bindings/net/ethernet.txt
+			  for details.
+
+- phy-mode		: String representing the connection to the designated
+			  PHY node specified by the 'phy-handle' property. See
+			  Documentation/devicetree/bindings/net/ethernet.txt
+			  for details.
+
+Optional subnodes:
+- fixed-link		: Fixed-link subnode describing a link to a non-MDIO
+			  managed entity. See
+			  Documentation/devicetree/bindings/net/fixed-link.txt
+			  for details.
+
 Example:
 
 	dsa@0 {
@@ -58,6 +74,7 @@ Example:
 			port@0 {
 				reg = <0>;
 				label = "lan1";
+				phy-handle = <&phy0>;
 			};
 
 			port@1 {