CVE-2018-3639 (powerpc)

Some versions of firmware will have a setting that can be configured
to disable the RFI flush, add support for it.

Fixes: 6e032b35 ("powerpc/powernv: Check device-tree for RFI flush settings")
(cherry picked from commit eb0a2d26)
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>