arm64: allwinner: pine64-plus: Enable dwmac-sun8i

The dwmac-sun8i hardware is present on the pine64 plus.
It uses an external PHY rtl8211e via RGMII.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>