arm64: dts: mediatek: mt7986: introduce ethernet nodes

Introduce ethernet nodes in mt7986 bindings in order to
enable mt7986a/mt7986b ethernet support.

Co-developed-by: Sam Shih <sam.shih@mediatek.com>
Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>