ppc/spapr: add 2.4 compat props

HW_COMPAT_2_4 will become non-empty: prepare for it.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Message-id: 1444991154-79217-3-git-send-email-cornelia.huck@de.ibm.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>