How to set SD clock

I am using the SDHC card on HPS Linux.
The theoretical value of operation clock is 50 MHz.
I think that it is set to 50 MHz and it is displayed also in the Linux start log as follows.

mmc_host mmc 0: Bus speed (slot 0) = 200000000 Hz (slot req 50000000 Hz, actual 50000000 HZ div = 2)

However, if you actually put the SDHC card in and operate it, it is operating at 12.5 MHz.
I confirmed it with an oscilloscope.

What kind of setting does it work at 50 MHz?

I solved this problem with reference to the following link.