Always enable built-in 2.5G PHY on MT7988 for now, so that it can be
used. In future it would be nice to be able to switch power and MDIO
access via address 0 at run-time in Linux, both, to be able to use
external PHYs at address 0 and to reduce power consumption on systems
not using the built-in 2.5G PHY.
Signed-off-by: NDaniel Golle <daniel@makrotopia.org>