dt-binding: phy: don't confuse with Ethernet phy properties

The generic PHY 'phys' property sometime appears in the same node with
the Ethernet PHY 'phy' or 'phy-handle' properties. Add a warning in
phy-bindings.txt to reduce confusion.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -34,7 +34,9 @@ PHY user node
 =============
 
 Required Properties:
-phys : the phandle for the PHY device (used by the PHY subsystem)
+phys : the phandle for the PHY device (used by the PHY subsystem; not to be
+       confused with the Ethernet specific 'phy' and 'phy-handle' properties,
+       see Documentation/devicetree/bindings/net/ethernet.txt for these)
 phy-names : the names of the PHY corresponding to the PHYs present in the
 	    *phys* phandle