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 <rogerq@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>