- Providing no compatible for an MDIO child node is considered to be equivalent
  to a c22 ethernet phy, so one must be provided. However, this node is then
  not automatically probed.

It cannot be automatically probed, since register 2 and 3 do not
contain an ID, which PHYs do. So you need to explicitly list in on the
MDIO bus, and when the of_mdiobus_register() is called, the device
will be instantiated.

Is it okay to provide a binding without a driver?

  If some code is required, where should this be put?
  Current devicetree structure:
    mdio-bus {
        compatible = "vendor,mdio";
        ...

        expander0: expander@0 {
            /*
             * Provide compatible for working registration of mdio device.
             * Device probing happens in gpio1 node.
             */
            compatible = "realtek,rtl8231-expander";
            reg = <0>;
        };

    };
    gpio1 : ext_gpio {
        compatible = "realtek,rtl8231-mdio";
        gpio-controller;
        ...
    };

I don't understand this split. Why not

     mdio-bus {
         compatible = "vendor,mdio";
         ...
 
         expander0: expander@0 {
             /*
              * Provide compatible for working registration of mdio device.
              * Device probing happens in gpio1 node.
              */
             compatible = "realtek,rtl8231-expander";
             reg = <0>;
	     gpio-controller;
         };
     };

You can list whatever properties you need in the node. Ethernet
switches have interrupt-controller, embedded MDIO busses with PHYs on
them etc.

- MFD driver:
  The RTL8231 is not just a GPIO expander, but also a pin controller and LED
  matrix controller. Regmap initialisation could probably be moved to a parent
  MFD, with gpio, led, and pinctrl cells. Is this a hard requirement if only a
  GPIO controller is provided?

You need to think about forward/backwards compatibility. You are
defining a binding now, which you need to keep. Do you see how an MFD
could be added without breaking backwards compatibility?

      Andrew