Add pgtable_t to remaining nommu architectures

The pte_fn_t in include/linux/mm.h make it necessary for all architectures
to define a pgtable_t type, even those that do not have an mmu.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>