mmu_has_feature(MMU_FTR_TYPE_RADIX) can be evaluated regardless of
CONFIG_PPC_RADIX_MMU.
When CONFIG_PPC_RADIX_MMU is not set, mmu_has_feature(MMU_FTR_TYPE_RADIX)
will evaluate to 'false' at build time because MMU_FTR_TYPE_RADIX
wont be included in MMU_FTRS_POSSIBLE.