gma: Introduce Generation type

It's the subset of `CPU_Type` that we have different compilation units
for. Also use it in the `Config` wherever we can decide something purely
on the `Generation` (i.e. don't mix `Gen` and `CPU` in expressions).

Change-Id: I5061021a80cd75ee3d7996ca343e6388b22bf341
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/libgfxinit/+/27059
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/configs/g45 b/configs/g45
index 6b54563..ef97850 100644
--- a/configs/g45
+++ b/configs/g45
@@ -1,3 +1,4 @@
+CONFIG_GFX_GMA_GENERATION	= G45
 CONFIG_GFX_GMA_CPU		= G45
 CONFIG_GFX_GMA_CPU_VARIANT	= Normal
 CONFIG_GFX_GMA_INTERNAL_PORT	= LVDS