Welcome, @şükrü krc, to the community!
First of all, DISCOVERY boards are intended to investigate the functions of the respective STM32 and are therefore already equipped with plenty of peripherals so that you can test this extensively without having to build your own board. For this reason, only a few GPIOs are free for alternative use on the DISCOVERY boards. In your particular case, the pins you need for the interfaces MII or RMII are already occupied, so you cannot retrofit the STM32F429I-DISC1 with an Ethernet PHY via RMII/MII.
If you want not only to try out the functions of the respective STM32, but to have almost complete freedom in development, you should consider a NUCLEO board instead, e.g. the NUCLEO-F429ZI.
Good luck!
Regards
/Peter