powerpc/powernv: Use the security flags in pnv_setup_rfi_flush()

powerpc/powernv: Use the security flags in pnv_setup_rfi_flush()

BugLink: https://bugs.launchpad.net/bugs/1760099
CVE-2017-5754

Now that we have the security flags we can significantly simplify the
code in pnv_setup_rfi_flush(), because we can use the flags instead of
checking device tree properties and because the security flags have
pessimistic defaults.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 37c0bdd00d3ae83369ab60a6712c28e11e6458d5 linux-next)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>