powerpc64: tell kernel when radix is not available

If CAS detects that radix is not supported, set radix_mmu to 0
to avoid the kernel trying to use it and panic.

Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)

(cherry picked from commit a23e18ea545675aca7551ef2395f6df40a3acb29)