we plan to use a Kinetis K60 (a MK60FN1M0VMD12, to be precise). It has an Ethernet interface, of which I like to use the MDIO interface only. But I need to run it at frequencies up to 4 MHz, as specifed for CFP MSA modules.
The K60 reference manual K60P144M150SF3RM.pdf only states "The MII_SPEED field must be programmed with a value to provide an MDC frequency of less than or equal to 2.5 MHz to be compliant with the IEEE 802.3 MII specification". The datasheet K60P144M150SF3.pdf does not mention any timing of the MDIO interface.
I presume that the HW would allow higher frequencies, but I don't find anything the maximum MDC clock rate in case IEEE compliance is not required.