mm: add architecture primitives for software dirty bit clearing

There are primitives to create and query the software dirty bits
in a pte or pmd. But the clearing of the software dirty bits is done
in common code with x86 specific page table functions.

Add the missing architecture primitives to clear the software dirty
bits to allow the feature to be used on non-x86 systems, e.g. the
s390 architecture.

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>