s390/mm: add set_pte_bit()/clear_pte_bit() helper functions

Add set_pte_bit()/clear_pte_bit() and set_pXd_bit()/clear_pXd_bit
helper functions which are supposed to be used if bits within
ptes/pXds are set/cleared.
The only point of these helper functions is to get more readable
code. This is quite similar to what arm64 has.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>