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/testers/memtest/memtest.cc

@@ -173,6 +173,7 @@ MemTest::completeRequest(PacketPtr pkt)
         safe_cast<MemTestSenderState *>(pkt->senderState);
 
     uint8_t *data = state->data;
+    // @todo: This should really be a const pointer
     uint8_t *pkt_data = pkt->getPtr<uint8_t>();
 
     //Remove the address from the list of outstanding

src/cpu/testers/rubytest/Check.cc

@@ -197,7 +197,7 @@ Check::initiateAction()
     pkt->dataDynamic(writeData);
 
     DPRINTF(RubyTest, "data 0x%x check 0x%x\n",
-            *(pkt->getPtr<uint8_t>()), *writeData);
+            *(pkt->getConstPtr<uint8_t>()), *writeData);
 
     // push the subblock onto the sender state.  The sequencer will
     // update the subblock on the return