asm-generic: introduce <asm-generic/pgtable-nop4d.h>

Like with pgtable-nopud.h for 4-level paging, this new header is base
for converting an architectures to properly folded p4d_t level.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>