mii_fdt: Add support for switch PHY node lookup

[kd]

Summary

Previously we would only search for a PHY xref in node of the miibus parent.
That didn't work very well with switches.
Fix that by searching through "ports" subnode, checking if any of its children have a valid PHY xref.
Since switches tend to have multiple ports we also have multiple candidates.
Use the PHY address read from mii_attach_args to find the right one.