powerpc/mm: allow hugepages without hugepd

In preparation of implementing huge pages on powerpc 8xx without hugepd,
enclose hugepd related code inside an ifdef CONFIG_ARCH_HAS_HUGEPD

This also allows removing some stubs.

Link: https://lkml.kernel.org/r/ada097ca8a4fa85a77f51719516ef2478800d77a.1719928057.git.christophe.leroy@csgroup.eu


Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>