s390/boot: remove pgtable_populate_end

setup_vmem() already calls populate for all online memory regions.
pgtable_populate_end() could be removed.

Also rename pgtable_populate_begin() to pgtable_populate_init().

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