net: ethernet: mtk_eth_soc: introduce support for mt7986 chipset

Add support for mt7986-eth driver available on mt7986 soc.

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