mediatek: simplify MaxLinear GPY PHY driver

mediatek: simplify MaxLinear GPY PHY driver

The mxl-gpy driver apparently was built in the assumption that SGMII
auto-negotiation is always switched on at the MAC. This may be true for
few rather recent drivers (why?), but certainly isn't for most drivers
unless 'managed = "in-band-status"' is set in device tree. Add patch to
the mediatek target which reduces mxl-gpy to behave more like an
ordinary PHY driver using out-of-band status.

This allows to use these PHYs without rate-adaptation which seems to be
at least partially broken/racy in some revisions of the PHY and/or
internal PHY firmware.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>