HI Alberto,
I am doing the same task on Beaglebone green wireless running on ubuntu 16.04 image but failed to enable the spi. I was initailly doing that on debian and that's working fine , but need to do on unbuntu.
Other things are working on ubuntu , like I enabled uart and i2c.
I even posted here and on stackoverflow .
If you can provide any help then it would be helpful to me .
Thanks,
Nimish