dt-bindings: add an explanation for internal phy-mode

When working on the Allwinner internal PHY, the first work was to use
the "internal" mode, but some answer was made my mail on what are really
internal mean for PHY.

This patch write that in the doc.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ethernet.txt

@@ -16,7 +16,8 @@ Documentation/devicetree/bindings/phy/phy-bindings.txt.
   Specification).
 - phy-mode: string, operation mode of the PHY interface. This is now a de-facto
   standard property; supported values are:
-  * "internal"
+  * "internal" (Internal means there is not a standard bus between the MAC and
+     the PHY, something proprietary is being used to embed the PHY in the MAC.)
   * "mii"
   * "gmii"
   * "sgmii"