mem-cache: Add MSHR debuging information

Add debug statment in MSHR and MSHRQueue class to track the
number of free MSHR each time a new one is allocated/deallocated.

Also track the allocation/deallocation of each MSHR target.

Change-Id: I2533e7660da1cde3052425f8db8852e59d463b42
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47041
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Daniel Carvalho <odanrc@yahoo.com.br>
Tested-by: kokoro <noreply+kokoro@google.com>

src/mem/cache/queue.hh

@@ -234,7 +234,8 @@ class Queue : public Drainable, public Named
      *
      * @param entry
      */
-    void deallocate(Entry *entry)
+    virtual void
+    deallocate(Entry *entry)
     {
         allocatedList.erase(entry->allocIter);
         freeList.push_front(entry);