There is the ar71 to ath79 porting guide thread, ... with nearly 1k of replies. There is a lot of info in there, but well-buried.
There are other threads for indivdual devices, but no generic "essential steps for getting basic device tree support going"-type page/wiki.
From the reply above from @slh, you can see, that the mach files provide a good starting point, but there is more to do after that.