net: mediatek: move MT7531 MMIO MDIO to dedicated driver

net: mediatek: move MT7531 MMIO MDIO to dedicated driver

In preparation for support of MDIO on AN7581, move the MT7531 MMIO logic
to a dedicated driver and permit usage of the mdio read/write function
to the mtk_eth driver.

This only affect MT7988 that can use MMIO operation to access the Switch
register. The MT7988 code is updated to make use of the external driver.

This permits Airoha driver to make use of DM_MDIO to bind for the MT7531
driver that have the same exact register.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>