net: phy: add driver for MediaTek SoC built-in GE PHYs

Some of MediaTek's Filogic SoCs come with built-in gigabit Ethernet
PHYs which require calibration data from the SoC's efuse.
Despite the similar design the driver doesn't share any code with the
existing mediatek-ge.c.
Add support for such PHYs by introducing a new driver with basic
support for MediaTek SoCs MT7981 and MT7988 built-in 1GE PHYs.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>