powerpc/mm: refactor pgtable freeing functions on nohash

pgtable_free() and others are identical on nohash/32 and 64,
so move them into asm/nohash/pgalloc.h

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>