dt-bindings: net: Restore sun8i dwmac binding

The original dwmac-sun8i DT bindings have some issue on how to handle
integrated PHY and was reverted in last RC of 4.13.
But now we have a solution so we need to get back that was reverted.

This patch restore dt-bindings documentation about dwmac-sun8i
This reverts commit 8aa33ec2 ("dt-bindings: net: Revert sun8i dwmac binding")

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