powerpc/mm: Compile out radix related functions if RADIX_MMU is disabled

Currently we depend on mmu_has_feature to evalute to zero based on
MMU_FTRS_POSSIBLE mask. In a later patch, we want to update
radix_enabled() to runtime update the conditional operation to a jump
instruction. This implies we cannot depend on MMU_FTRS_POSSIBLE mask.
Instead define radix_enabled to return 0 if RADIX_MMU is not enabled.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>