Is there any documentation on how to write xxmatch() and xxattach()?
I see pretty much the same thing in all of the xxmatch() routines in
/sys/arch/sparc/dev, but I don't quite get what they're talking about.
What are the parameters? What is supposed to happen in xxattach() with
regards to mapping registers and interrupts and the like?

The routines in the cdevsw I pretty much have figured out. It's just
the sparc specific (and in particular sbus specific) stuff I need a
bit of a pointer on.