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: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>