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/abstract_mem.cc

@@ -79,6 +79,8 @@ AbstractMemory::setBackingStore(uint8_t* pmem_addr)
 void
 AbstractMemory::regStats()
 {
+    MemObject::regStats();
+
     using namespace Stats;
 
     assert(system());

src/mem/cache/base.cc

@@ -176,6 +176,8 @@ BaseCache::inRange(Addr addr) const
 void
 BaseCache::regStats()
 {
+    MemObject::regStats();
+
     using namespace Stats;
 
     // Hit statistics

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")

src/mem/cache/tags/base.cc

@@ -71,7 +71,10 @@ BaseTags::setCache(BaseCache *_cache)
 void
 BaseTags::regStats()
 {
+    ClockedObject::regStats();
+
     using namespace Stats;
+
     replacements
         .init(maxThreadsPerCPU)
         .name(name() + ".replacements")

src/mem/comm_monitor.cc

@@ -384,6 +384,8 @@ CommMonitor::recvRangeChange()
 void
 CommMonitor::regStats()
 {
+    MemObject::regStats();
+
     // Initialise all the monitor stats
     using namespace Stats;
 

src/mem/xbar.cc

@@ -541,6 +541,8 @@ BaseXBar::getAddrRanges() const
 void
 BaseXBar::regStats()
 {
+    ClockedObject::regStats();
+
     using namespace Stats;
 
     transDist