mediatek: switch to pending XFI 10G Ethernet drivers

Replace previous patch adding paths and SerDes modes with patch series
pending upstream adding dedicated drivers for XFI T-PHY and USXGMII PCS,
extends LynxI PCS to be a standalone platform driver and as a consequence
makes much less changes to the actual Ethernet driver mtk_eth_soc.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>