net: ethernet: mtk_eth_soc: introduce MTK_NETSYS_V2 support

Introduce MTK_NETSYS_V2 support. MTK_NETSYS_V2 defines 32B TX/RX DMA
descriptors.
This is a preliminary patch to add mt7986 ethernet support.

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>