powerpc/powernv: Restrict the busdma tag to only POWER8

It seems this tag is causing problems on POWER9 systems. Since no POWER9 user
has encountered the problem fixed by r339589 just restrict it to POWER8 for now.
A better fix will likely be to update powerpc/busdma_machdep.c to handle the
window correctly.

Reported by: mmacy, others