phy: core: Add phy-supply to DT binding documentation

phy-supply is a phandle to the regulator that provides power to the
PHY. This regulator is managed during the PHY power on/off sequence
by the phy core driver.
Signed-off-by: NRoger Quadros <rogerq@ti.com>
Signed-off-by: NKishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/phy-bindings.txt

@@ -10,6 +10,10 @@ Required Properties:
 		provider can use the values in cells to find the appropriate
 		PHY.
 
+Optional Properties:
+phy-supply:	Phandle to a regulator that provides power to the PHY. This
+		regulator will be managed during the PHY power on/off sequence.
+
 For example:
 
 phys: phy {