s390/mm: Convert pgste_val() into function

s390/mm: Convert pgste_val() into function

Similar to all other *_val() functions convert the last remaining
architecture specific mm primitive pgste_val() into a function.

Add set_pgste_bit() and clear_pgste_bit() helper functions which allow to
clear and set pgste bits. This is also similar to e.g. set_pte_bit() and
other helper functions.

Acked-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>