mem-cache: add pfIssued stat in MultiPrefetcher

Count issued prefetches for each prefetchter in a MultiPrefetcher

Change-Id: If03fb0669af9bb92ce9cf210b6201a9719a7c771
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47600
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Daniel Carvalho <odanrc@yahoo.com.br>

src/mem/cache/prefetch/multi.cc

@@ -77,6 +77,8 @@ Multi::getPacket()
         if (pf->nextPrefetchReadyTime() <= curTick()) {
             PacketPtr pkt = pf->getPacket();
             panic_if(!pkt, "Prefetcher is ready but didn't return a packet.");
+            prefetchStats.pfIssued++;
+            issuedPrefetches++;
             return pkt;
         }
     }