arm64: allwinner: a64: add ethernet0 alias for SoPine EMAC node

The SoPine official baseboard uses the A64 chip's EMAC to provide an
Ethernet link.

Add the ethernet0 alias in the device tree, in order to let U-Boot
generate a MAC address from the chip's SID.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>