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: Roger Quadros <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

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

···
 		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 {