Hello, 
I try to activate the spi on my raspberry3 using a fedora 26. 
I tried these options:
device_tree_param=spi=on
dtoverlay=spi-bcm2708

I use this kernel:
4.12.5-300.fc26.armv7hl

I see that the support in kernel seems already inside:
CONFIG_SPI=y
# CONFIG_SPI_DEBUG is not set
CONFIG_SPI_MASTER=y


But I never see the devices /dev/spi* appearing. 
I can't see any messages regarding spi in dmesg. 

I tested with a raspbian and I see them correctly. 
I think I miss somethiing. 
Could you help. 
Thanks.