dt-bindings: net: dwmac-sun8i: update documentation about integrated PHY

This patch add documentation about the MDIO switch used on sun8i-h3-emac
for integrated PHY.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>