wow !
in fact it was much more simple ..
I was playing with a raspberry 3B and not a 3B+ so the dtb file was not the good one :)
Finally I have this :
cd /proc/device-tree/soc
ls -dal spi*
drwxr-xr-x. 2 root root 0 Mar 5 23:16 spi@7e204000
drwxr-xr-x. 2 root root 0 Mar 5 23:16 spi@7e215080
drwxr-xr-x. 2 root root 0 Mar 5 23:16 spi@7e2150c0
cat spi*/status
okay
disabled
okay
but no device in /dev..
do you have any hints ?
Thanks