s390/Kconfig: sort list of arch selected config options

Just like on other architectures. The intention is that this will reduce
merge conflicts if new config options get added in sorted order as well.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>