MT7981 and the upcoming MT7988 have built-in Gigabit Ethernet PHYs.
While they share some design properties with the PHYs present in
MT753x, they do need calibration data from the SoC's efuse.
Add driver to support them. Upstreaming it is planned, but there are
still some ongoing discussions with MediaTek.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>