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/dev/storage/ide_disk.cc

@@ -392,6 +392,8 @@ IdeDisk::doDmaDataRead()
 void
 IdeDisk::regStats()
 {
+    SimObject::regStats();
+
     using namespace Stats;
     dmaReadFullPages
         .name(name() + ".dma_read_full_pages")