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/mem/cache/cache.hh

@@ -287,7 +287,7 @@ class Cache : public BaseCache
                                bool pending_downgrade = false);
     bool satisfyMSHR(MSHR *mshr, PacketPtr pkt, BlkType *blk);
 
-    void doTimingSupplyResponse(PacketPtr req_pkt, uint8_t *blk_data,
+    void doTimingSupplyResponse(PacketPtr req_pkt, const uint8_t *blk_data,
                                 bool already_copied, bool pending_inval);
 
     /**