In order to make this work, I created a pseudo-PHY driver to deal with
Macronix chips that use the built-in NWAY support and symbol mode port.
This is actually all of them, with the exception of the original MX98713
which presents its NWAY support via the MII serial interface.
The mxphy driver actually manipulates the controller registers directly
rather than using the miibus_readreg()/miibus_writereg() bus interface
since there are no MII registers to read. The mx driver itself pretends
that the NWAY interface is a PHY locayed at MII address 31 for the sole
purpose of allowing the mxphy_probe() routine to know when it needs to
attach to a host controller.
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