sim: Call regStats of base-class as well

We want to extend the stats of objects hierarchically and thus it is necessary
to register the statistics of the base-class(es), as well.  For now, these are
empty, but generic stats will be added there.

Patch originally provided by Akash Bagdia at ARM Ltd.

src/mem/cache/prefetch/base.cc

@@ -74,6 +74,8 @@ BasePrefetcher::setCache(BaseCache *_cache)
 void
 BasePrefetcher::regStats()
 {
+    ClockedObject::regStats();
+
     pfIssued
         .name(name() + ".num_hwpf_issued")
         .desc("number of hwpf issued")