dts: stmmac: Document the clocks property in the stmmac base document

Add the clocks and clock-names property to the base stmmac dts bindings
document.
Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/net/stmmac.txt

@@ -34,6 +34,10 @@ Optional properties:
 	reset phandle is given
 - max-frame-size:	Maximum Transfer Unit (IEEE defined MTU), rather
 			than the maximum frame size.
+- clocks: If present, the first clock should be the GMAC main clock,
+  further clocks may be specified in derived bindings.
+- clocks-names: One name for each entry in the clocks property, the
+  first one should be "stmmaceth".
 
 Examples:
 
@@ -46,4 +50,6 @@ Examples:
 		mac-address = [000000000000]; /* Filled in by U-Boot */
 		max-frame-size = <3800>;
 		phy-mode = "gmii";
+		clocks = <&clock>;
+		clock-names = "stmmaceth">;
 	};