gma: Add flag to use GMCH GMBUS registers

Change-Id: Id5d115c7a4711f634171e6e163439ebab4ee6076
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/25405
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/common/hw-gfx-gma-config.ads.template b/common/hw-gfx-gma-config.ads.template
index 2349272..b38201e 100644
--- a/common/hw-gfx-gma-config.ads.template
+++ b/common/hw-gfx-gma-config.ads.template
@@ -110,6 +110,7 @@
    ----- GMBUS: -----------
    Ungate_GMBUS_Unit_Level : constant Boolean := CPU >= Skylake;
    GMBUS_Alternative_Pins  : constant Boolean := CPU = Broxton;
+   Has_PCH_GMBUS           : constant Boolean := CPU >= Ironlake;
 
    ----- Power: -----------
    Has_IPS                 : constant Boolean := (CPU = Haswell and