[S390] Fix register usage description.

Fix description of register usage as pointed out by Andreas Krebbel.
Since this document is completely outdated and would need a lot of
fixing, it might be worth considering to get rid of it...

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