net: ethernet: mtk_eth_soc: set MDIO bus clock frequency

Set MDIO bus clock frequency and allow setting a custom maximum
frequency from device tree.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>