1) The K64FX512 uses with for specific ethernet PHY ?
The FRDM-K64 uses the KSZ8081RNA and the example works with this PHY, if you're using a different one you will need to modify the configurations to match the PHY that you're using.
2) The 50MHz clock is from IC+175D chip that I use PHY and connect to pin50, PTA18/EXTAL0, the mode is set RMII.
How can I set this clock on my program?
If I understanding correctly, you're using an external reference clock to supply the MCU, if this is the case, the lwip examples already set this to work with an external clock, in the case of the FRDM this is supplied by the PHY.