dt-bindings: net: mdio: remove compatible string from example

Remove vendor specific compatible string from example, otherwise DT YAML
schemas validation may trigger warnings specific to TI ti,davinci_mdio
and not to the generic MDIO example.

For example, the "bus_freq" is required for davinci_mdio, but not required for
generic mdio example. As result following warning will be produced:
 mdio.example.dt.yaml: mdio@5c030000: 'bus_freq' is a required property
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/net/mdio.yaml

@@ -56,7 +56,6 @@ patternProperties:
 examples:
   - |
     davinci_mdio: mdio@5c030000 {
-        compatible = "ti,davinci_mdio";
         reg = <0x5c030000 0x1000>;
         #address-cells = <1>;
         #size-cells = <0>;