changing include/asm-generic/pgtable.h for non-mmu

There are some parts of include/asm-generic/pgtable.h that are relevant to
the non-mmu architectures.  To make it easier to include this from them I
would like to ifdef the relevant parts.

Without this there is a handful of functions that are referenced in here
that are not defined on many non-mmu architectures.  They could be defined
out of course, as an alternative approach.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>