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/arch/arm/table_walker.cc

@@ -2122,6 +2122,8 @@ TableWalker::pageSizeNtoStatBin(uint8_t N)
 void
 TableWalker::regStats()
 {
+    ClockedObject::regStats();
+
     statWalks
         .name(name() + ".walks")
         .desc("Table walker walks requested")

src/arch/arm/tlb.cc

@@ -417,6 +417,7 @@ TLB::unserialize(CheckpointIn &cp)
 void
 TLB::regStats()
 {
+    BaseTLB::regStats();
     instHits
         .name(name() + ".inst_hits")
         .desc("ITB inst hits")