dt-bindings: net: don't confuse with generic PHY property

This complements commit 9a94b3a4 (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
ethernet.txt to reduce confusion.
Signed-off-by: NBaruch Siach <baruch@tkos.co.il>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ethernet.txt

 The following properties are common to the Ethernet controllers:
 
+NOTE: All 'phy*' properties documented below are Ethernet specific. For the
+generic PHY 'phys' property, see
+Documentation/devicetree/bindings/phy/phy-bindings.txt.
+
 - local-mac-address: array of 6 bytes, specifies the MAC address that was
   assigned to the network device;
 - mac-address: array of 6 bytes, specifies the MAC address that was last used by