arm64: dts: allwinner: a64: NanoPi-A64: Add Ethernet

The NanoPi-A64 has the usual Realtek Gbit PHY connected to the EMAC,
so add the respective nodes to the DT. The PHY is powered by the
VDD_SYS_3.3V line, which is always on.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>