Hey there, I found a solution for my problem.
I had to download this precompiled file (firmware/spi0-2cs.dtbo at master · raspberrypi/firmware · GitHub) and put it into my SD card on folder /boot/overlays .
Then I edited the file /boot/config.txt (only the last line). The last line must be like
dtoverlay=spi0-2cs,cs1_pin=<another free pin different from 7>
In my case I used pin 31 for Raspberry Pi 3 (GPIO6) which is unused and it worked.