mem: Add parameter to reserve MSHR entries for demand access

Adds a new parameter that reserves some number of MSHR entries for demand
accesses.  This helps prevent prefetchers from taking all MSHRs, forcing demand
requests from the CPU to stall.

src/mem/cache/cache_impl.hh

@@ -1841,7 +1841,7 @@ Cache<TagStore>::getNextMSHR()
 
     // fall through... no pending requests.  Try a prefetch.
     assert(!miss_mshr && !write_mshr);
-    if (prefetcher && !mshrQueue.isFull()) {
+    if (prefetcher && mshrQueue.canPrefetch()) {
         // If we have a miss queue slot, we can try a prefetch
         PacketPtr pkt = prefetcher->getPacket();
         if (pkt) {