dt-bindings: net: phy: Document support for LEDs node

Document support for LEDs node in phy and add an example for it.
PHY LED will have to match led pattern and should be treated as a
generic led.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>