net: ethernet: mtk_eth_soc: introduce device register map

Introduce reg_map structure to add the capability to support different
register definitions. Move register definitions in mtk_regmap structure.
This is a preliminary patch to introduce 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>