dt-bindings: net: Add Cortina device tree bindings

Add device tree description info for Cortina 10G phy devices.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>