On ppc64 host, recursion into pc-bios/spapr-rtas/ fails for
out-of-tree builds. Add missing dir and symlink.

Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Signed-off-by: NAlexander Graf <agraf@suse.de>