dt-bindings: net: ipq4019-mdio: document required clock-names

IPQ5018, IPQ6018 and IPQ8074 require clock-names to be set as driver is
requesting the clock based on it and not index, so document that and make
it required for the listed SoC-s.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221114194734.3287854-4-robimarko@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>