Documentation: devicetree: add a new optional property for port mac address

Add documentation for a new optional property local-mac-address which
is described in ethernet.txt.
Signed-off-by: Nxiaofeis <xiaofeis@codeaurora.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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

@@ -71,6 +71,10 @@ properties, described in binding documents:
 			  Documentation/devicetree/bindings/net/fixed-link.txt
 			  for details.
 
+- local-mac-address	: See
+			  Documentation/devicetree/bindings/net/ethernet.txt
+			  for details.
+
 Example
 
 The following example shows three switches on three MDIO busses,
@@ -97,6 +101,7 @@ linked into one DSA cluster.
 			port@1 {
 				reg = <1>;
 				label = "lan1";
+				local-mac-address = [00 00 00 00 00 00];
 			};
 
 			port@2 {