gma: Add procedure to power up legacy VGA block

Calling this is necessary for VGA text mode on at least Skylake, where
the legacy VGA registers are inaccessible otherwise.

Change-Id: I48ba1738bcc7babd4e666e5266f775dcd06b2a3f
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/21323
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: Nico Huber <nico.h@gmx.de>
diff --git a/common/hw-gfx-gma.ads b/common/hw-gfx-gma.ads
index 3771804..cd992b8 100644
--- a/common/hw-gfx-gma.ads
+++ b/common/hw-gfx-gma.ads
@@ -80,6 +80,12 @@
       Global => (Input => Init_State);
    pragma Warnings (GNATprove, On, "unused variable ""Write_Delay""");
 
+   pragma Warnings (GNATprove, Off, "subprogram ""Power_Up_VGA"" has no effect",
+                    Reason => "Effect depends on the platform compiled for");
+   procedure Power_Up_VGA
+   with
+      Pre => Is_Initialized;
+
    procedure Update_Outputs (Configs : Pipe_Configs);
 
    pragma Warnings (GNATprove, Off, "subprogram ""Dump_Configs"" has no effect",