powerpc/mm: Add radix callbacks to pte accessors

For those pte accessors, that operate on a different set of pte bits
between hash/radix, we add a generic variant that does a conditional
to hash linux or radix variant.

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