Add LSU stats (#30)

Added a few additional LSU statistics. 

Output from running dhrystone trace:

```
        top.cpu.core0.lsu
            lsu_insts_dispatched                              = 5850001
            stores_retired                                    = 5850001
            lsu_insts_issued                                  = 11700073
            lsu_insts_completed                               = 5849999
            lsu_flushes                                       = 0
            tlb_hits                                          = 5850001
            tlb_misses                                        = 16
            dl1_cache_hits                                    = 5850000
            dl1_cache_misses                                  = 55
            biu_reqs                                          = 19

          top.cpu.core0.lsu.tlb
              tlb_hits                                        = 5850001
```

I will further look into the stats to make sure that they make sense.
But the added stats can now be used for the purpose of the demo.