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>

Documentation/devicetree/bindings/net/ethernet.txt

@@ -11,6 +11,7 @@ The following properties are common to the Ethernet controllers:
   the maximum frame size (there's contradiction in ePAPR).
 - phy-mode: string, operation mode of the PHY interface. This is now a de-facto
   standard property; supported values are:
+  * "internal"
   * "mii"
   * "gmii"
   * "sgmii"