> Are there any examples how to split common and cpu specific parts cleanly?

No, I'm afraid.  "Good SoC" will survive longer and accumulate (very) much
more and more as time goes by.  It's rather inconsistent and cumbersome the
way NetBSD handles MIPS/PPC SoC, however, they're SoC; designer can do
anything as their desire go, that is, HW design itself is not consistent nor
sharable, and not avoidable to try hard N-way maintenance in practice.

Toru Nishimura/ALKYL Technology