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>