What's the closest SOF branch to linux-next or sound/for-next?
Our i.MX8 boards support was only recently added in various trees and it is currently only in linux-next.
Every week, I select patches from Takashi and Mark's for-next tree into the backport/v4.20-intel-audio branch and merge them on top of the development branch topic/sof-dev. I also squash all SOF patches in topic/sof-dev-rebase. You can take that branch and all the changes apply it on top of linux-next, it should "just work", e.g. git cherry-pick backport/v4.20-intel-audio..topic/sof-dev-rebase.
If you want to have a non-rebasing tree with the history, it's a bit more complicated since there are a small number of conflicts, e.g. git merge topic/sof-dev mostly works except where it doesn't.