configs: Add default to max_cu_tokens CLA

Fixes nightly tests

Change-Id: Ibad2e8a52afb62b0605ffa7ca958e378df799dae
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62653
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>

configs/example/apu_se.py

@@ -252,6 +252,7 @@ parser.add_argument(
 parser.add_argument(
     "--max-cu-tokens",
     type=int,
+    default=4,
     help="Number of coalescer tokens per CU",
 )
 parser.add_argument(