gma: Give constants depending on Config.CPU* a type

To reduce elaboration time dependencies, turn number constants into
variable constants by giving them a type. This will allow us to use
the same code for configurations with constant and non-constant
`GMA.Config.CPU*`.

Change-Id: I0b671cb18701d0ecae118e023e0fd21be5095da0
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/libgfxinit/+/27063
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/common/hw-gfx-gma.adb b/common/hw-gfx-gma.adb
index 0a80d6b..dfe986f 100644
--- a/common/hw-gfx-gma.adb
+++ b/common/hw-gfx-gma.adb
@@ -695,7 +695,7 @@
    with
       Pre => Is_Initialized
    is
-      GGC_Reg : constant :=
+      GGC_Reg : constant PCI.Index :=
         (if Config.Gen_G45 or Config.CPU_Ironlake then 16#52# else 16#50#);
       GGC : Word16;
    begin