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

src/mem/cache/SConscript

@@ -30,7 +30,7 @@
 
 Import('*')
 
-SimObject('BaseCache.py')
+SimObject('Cache.py')
 
 Source('base.cc')
 Source('cache.cc')