mem: Add const getters for write packet data

This patch takes a first step in tightening up how we use the data
pointer in write packets. A const getter is added for the pointer
itself (getConstPtr), and a number of member functions are also made
const accordingly. In a range of places throughout the memory system
the new member is used.

The patch also removes the unused isReadWrite function.

src/cpu/o3/fetch_impl.hh

@@ -388,7 +388,7 @@ DefaultFetch<Impl>::processCacheCompletion(PacketPtr pkt)
         return;
     }
 
-    memcpy(fetchBuffer[tid], pkt->getPtr<uint8_t>(), fetchBufferSize);
+    memcpy(fetchBuffer[tid], pkt->getConstPtr<uint8_t>(), fetchBufferSize);
     fetchBufferValid[tid] = true;
 
     // Wake up the CPU (if it went to sleep and was waiting on