dt-bindings: Add "internal" as a valid 'phy-mode' property

A number of Ethernet MACs have internal Ethernet PHYs and the internal
wiring makes it so that this knowledge needs to be available using the
standard 'phy-mode' property.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>