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Add driver for MediaTek's XFI T-PHY which can be found in the MT7988
SoC. The XFI T-PHY is a 10 Gigabit/s Ethernet SerDes PHY with muxes on
the internal side to be used with either USXGMII PCS or LynxI PCS,
depending on the selected PHY interface mode.

The PHY can operates only in PHY_MODE_ETHERNET, the submode is one of
PHY_INTERFACE_MODE_* corresponding to the supported modes:

 * USXGMII                 \
 * 10GBase-R                }- USXGMII PCS - XGDM  \
 * 5GBase-R                /                        \
                                                     }- Ethernet MAC
 * 2500Base-X              \                        /
 * 1000Base-X               }- LynxI PCS - GDM     /
 * Cisco SGMII (MAC side)  /

I chose the name XFI T-PHY because names of functions dealing with the
phy in the vendor driver are prefixed "xfi_pextp_".

The register space used by the phy is called "pextp" in the vendor
sources, which could be read as "_P_CI _ex_press _T_-_P_hy", and...