[S390] Move psw_set_key.

Move psw_set_key() from ptrace.h to processor.h which is a more
suitable place for it. In addition the moves makes the function
invisible to user space.

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