powerpc/nohash: Update 64K nohash config to have 32 pte fragement

They don't need to track 4k subpage slot details and hence don't need
second half of pgtable_t.

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