resources: Add a default value for the PrefetchCache constructor

The spec2006 gem5 configs currently doesn't work due to the
PrefetchCache constructor is called without specifying `options`.
Since `options` is not a required parameter, it's safe to set
`None` as the default value for this parameter.

Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Change-Id: Iacfeb7d1368af80925bdd5e9364807cadf34a568
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-resources/+/37235
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
diff --git a/src/spec-2006/configs/system/caches.py b/src/spec-2006/configs/system/caches.py
index dc26d77..f15b741 100644
--- a/src/spec-2006/configs/system/caches.py
+++ b/src/spec-2006/configs/system/caches.py
@@ -44,7 +44,7 @@
 
 class PrefetchCache(Cache):
 
-    def __init__(self, options):
+    def __init__(self, options = None):
         super(PrefetchCache, self).__init__()
         if not options or options.no_prefetchers:
             return