mm: make the __PAGETABLE_PxD_FOLDED defines non-empty

Change the currently empty defines for __PAGETABLE_PMD_FOLDED,
__PAGETABLE_PUD_FOLDED and __PAGETABLE_P4D_FOLDED to return 1.
This makes it possible to use __is_defined() to test if the
preprocessor define exists.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>