Finally I was able to finish the bisect with some help from slyfox from
#gentoo-powerpc. At last...

I made a second bisect in search of the fix for the "Couldn't allocate pmd
pagetable caches" error, which I found in commit
bf5ca68dd2eef59a936969e802d811bdac4709c2 (powerpc: Fix pgtable pmd cache init).
I applied this commit via git cherry-pick --no-commit to my first bisect and
was able to continue withouth further skips. So, the final candidate for the
"ring 0" error is:

commit 60db3a4d8cc9073cf56264785197ba75ee1caca4 (refs/bisect/bad)
Author: Sinan Kaya <okaya@codeaurora.org>

    PCI: Enable PCIe Extended Tags if supported