Documentation: macb: Document phy-handle binding

Document the existence of the optional binding, directing to the
general ethernet document that describes this binding.

Signed-off-by: Brad Mouring <brad.mouring@ni.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/macb.txt

@@ -29,6 +29,7 @@ Optional properties for PHY child node:
 - reset-gpios : Should specify the gpio for phy reset
 - magic-packet : If present, indicates that the hardware supports waking
   up via magic packet.
+- phy-handle : see ethernet.txt file in the same directory
 
 Examples: