powerpc/powernv: Support firmware disable of RFI flush

commit eb0a2d2620ae431c543963c8c7f08f597366fc60 upstream.

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

Fixes: 6e032b350cd1 ("powerpc/powernv: Check device-tree for RFI flush settings")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>