arm: sun8i: orangepi-zero: Enable dwmac-sun8i

The dwmac-sun8i hardware is present on the Orange PI Zero.
It uses the internal PHY.

This patch create the needed emac node.

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