gma hsw+: Add default value for HDMI buffer levels

Add a configuration option to override the default HDMI translation.
It's not exported yet, as we've never seen a non-default value being
used. So this is just to have a common place for the defaults.

Also sets the recommended default value for Broxton as defined in the
PRM.

Change-Id: I59fae0fb4f444d9193a98b6a0edf337ecbba3b62
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/20165
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/common/hw-gfx-gma.ads b/common/hw-gfx-gma.ads
index 8d51960..c8fc398 100644
--- a/common/hw-gfx-gma.ads
+++ b/common/hw-gfx-gma.ads
@@ -1,5 +1,5 @@
 --
--- Copyright (C) 2015-2016 secunet Security Networks AG
+-- Copyright (C) 2015-2017 secunet Security Networks AG
 --
 -- This program is free software; you can redistribute it and/or modify
 -- it under the terms of the GNU General Public License as published by
@@ -133,4 +133,8 @@
 
    type DP_Port is (DP_A, DP_B, DP_C, DP_D);
 
+   ----------------------------------------------------------------------------
+
+   subtype DDI_HDMI_Buf_Trans_Range is Integer range 0 .. 11;
+
 end HW.GFX.GMA;