PHY seems to be one of those cases where it's okay to have the
compatible property but also okay to not have it.

Correct. They are like PCI or USB devices. You can ask it, what are
you? There are two registers in standard locations which give you a
vendor and product ID. We use that to find the correct driver.

You only need a compatible when things are not so simple.

1) The IDs are wrong. Some silicon vendors do stupid things

2) Chicken/egg problems, you cannot read the ID registers until you
   load the driver and some resource is enabled.

3) It is a C45 devices, e.g. part of clause 45 of 802.3, which
   requires a different protocol to be talked over the bus. So the
   compatible string tells you to talk C45 to get the IDs.

4) It is not a PHY, but some sort of other MDIO device, and hence
   there are no ID registers.

Andrew