arm64: dts: allwinner: A64: Restore EMAC changes

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 arm64 DT about dwmac-sun8i for A64
This reverts commit 87e1f5e8

 ("arm64: dts: allwinner: Revert EMAC changes")

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