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>