ARM64: dts: mediatek: add the ethernet node to mt8516.dtsi

Add the Ethernet MAC node to mt8516.dtsi. This defines parameters common
to all the boards based on this SoC.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>