dt-bindings: net: ipq4019-mdio: document IPQ6018 compatible

Document IPQ6018 compatible that is already being used in the DTS along
with the fallback IPQ4019 compatible as driver itself only gets probed
on IPQ4019 and IPQ5018 compatibles.

This is also required in order to specify which platform require clock to
be defined and validate it in schema.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221114194734.3287854-1-robimarko@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>