CVE-2018-3639 (powerpc)

Now that we have feature flags for security related things, set or
clear them based on what we see in the device tree provided by
firmware.

(cherry picked from commit 77addf6e)
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>