powerpc/mm: Free up _PAGE_COHERENCE for numa fault use later

Set  memory coherence always on hash64 config. If
a platform cannot have memory coherence always set they
can infer that from _PAGE_NO_CACHE and _PAGE_WRITETHRU
like in lpar. So we dont' really need a separate bit
for tracking _PAGE_COHERENCE.

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>