Documentation: net: fsl-fec.txt: Add phy-supply entry

phy-supply is an optional property of the fec driver, so add it to the binding
documentation.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

Documentation/devicetree/bindings/net/fsl-fec.txt

@@ -15,6 +15,7 @@ Optional properties:
   only if property "phy-reset-gpios" is available.  Missing the property
   will have the duration be 1 millisecond.  Numbers greater than 1000 are
   invalid and 1 millisecond will be used instead.
+- phy-supply: regulator that powers the Ethernet PHY.
 
 Example:
 
@@ -25,4 +26,5 @@ ethernet@83fec000 {
 	phy-mode = "mii";
 	phy-reset-gpios = <&gpio2 14 0>; /* GPIO2_14 */
 	local-mac-address = [00 04 9F 01 1B B9];
+	phy-supply = <&reg_fec_supply>;
 };