arm: dts: sunxi: update A64 to new EMAC binding

The U-Boot driver for the sun8i-emac was using some preliminary DT
binding. Now since Linux got its own driver in v4.15 and our driver
can now cope with both bindings, let's convert the DT nodes used for the
Pine64+ board over to the new bindings used by the kernel.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>