powerpc/mm: Enable _PAGE_NUMA for book3s

We steal the _PAGE_COHERENCE bit and use that for indicating NUMA ptes.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>