dt-bindings: net: mscc-miim: add clock and clock-frequency

Add the (optional) clock input of the MDIO controller and indicate that
the common clock-frequency property is supported. The driver can use it
to set the desired MDIO bus frequency.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>