Add bindings for the MediaTek XFI Ethernet SerDes T-PHY found in the
MediaTek MT7988 SoC which can operate at various interfaces modes:
via USXGMII PCS:
* USXGMII
* 10GBase-R
* 5GBase-R
via LynxI SGMII PCS:
* 2500Base-X
* 1000Base-X
* Cisco SGMII (MAC side)
Signed-off-by: Daniel Golle <daniel@xxxxxxxxxxxxxx>