s390x: add cpacf.h from Linux

Add arch/s390/include/asm/cpacf.h from Linux v4.13.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170919145033.16959-3-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>