mem: Add explicit Cache subclass and make BaseCache abstract

Open up for other subclasses to BaseCache and transition to using the
explicit Cache subclass.

--HG--
rename : src/mem/cache/BaseCache.py => src/mem/cache/Cache.py

configs/splash2/cluster.py

@@ -137,7 +137,7 @@ class Water_spatial(LiveProcess):
 # Base L1 Cache Definition
 # ====================
 
-class L1(BaseCache):
+class L1(Cache):
     latency = options.l1latency
     mshrs = 12
     tgts_per_mshr = 8
@@ -146,7 +146,7 @@ class L1(BaseCache):
 # Base L2 Cache Definition
 # ----------------------
 
-class L2(BaseCache):
+class L2(Cache):
     latency = options.l2latency
     mshrs = 92
     tgts_per_mshr = 16