arm64: allwinner: pine64: Enable dwmac-sun8i

The dwmac-sun8i hardware is present on the pine64
It uses an external PHY via RMII.

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