powerpc/powernv: Support firmware disable of RFI flush

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>

arch/powerpc/platforms/powernv/setup.c

@@ -79,6 +79,10 @@ static void pnv_setup_rfi_flush(void)
 		if (np && of_property_read_bool(np, "disabled"))
 			enable--;
 
+		np = of_get_child_by_name(fw_features, "speculation-policy-favor-security");
+		if (np && of_property_read_bool(np, "disabled"))
+			enable = 0;
+
 		of_node_put(np);
 		of_node_put(fw_features);
 	}