net: mtk_eth_soc: provide mtk_sgmii_config()

Provide mtk_sgmii_config() to wrap up the decisions about which SGMII
configuration will be called.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>