s390: ignore pkey system calls

Ignore the pkey systems calls since they don't make any sense on s390.
In addition any user could trigger a warning if issueing the pkey_free
system call, if it would be wired up on a system without pkey support.

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