net: mtk_eth_soc: partially convert to phylink_pcs

Partially convert mtk_eth_soc to phylink_pcs, moving the configuration,
link up and AN restart over. However, it seems mac_pcs_get_state()
doesn't actually get the state from the PCS, so we can't convert that
over without a better understanding of the hardware.

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