configs: Fix L3Cache instantiation in lat_mem_rd.py

This changeset updates the lat_mem_rd.py to configure the L3Cache
using the split tag_latency, data_latency parameters.

Change-Id: I8bc41d5f7664111bdda0972356d1a17762aa77e5
Reviewed-on: https://gem5-review.googlesource.com/8288
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

configs/dram/lat_mem_rd.py

@@ -264,7 +264,9 @@ from common.Caches import *
 # a starting point for an L3 cache
 class L3Cache(Cache):
     assoc = 16
-    hit_latency = 40
+    tag_latency = 20
+    data_latency = 20
+    sequential_access = True
     response_latency = 40
     mshrs = 32
     tgts_per_mshr = 12