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/mshr_queue.hh

@@ -96,6 +96,11 @@ class MSHRQueue : public Queue<MSHR>
     MSHR *allocate(Addr blk_addr, unsigned blk_size, PacketPtr pkt,
                    Tick when_ready, Counter order, bool alloc_on_fill);
 
+    /**
+     * Deallocate a MSHR and its targets
+     */
+    void deallocate(MSHR *mshr) override;
+
     /**
      * Moves the MSHR to the front of the pending list if it is not
      * in service.