net: ethernet: mtk_eth_soc: implement Clause 45 MDIO access

Implement read and write access to IEEE 802.3 Clause 45 Ethernet
phy registers while making use of new mdiobus_c45_regad and
mdiobus_c45_devad helpers.

Tested on the Ubiquiti UniFi 6 LR access point featuring
MediaTek MT7622BV WiSoC with Aquantia AQR112C.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>