mem: Add snoop filter to SystemXBar by default

This patch changes the default behaviour of the SystemXBar, adding a
snoop filter. With the recent updates to the snoop filter allocation
behaviour this change no longer causes problems for the regressions
without caches.

Change-Id: Ibe0cd437b71b2ede9002384126553679acc69cc1
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Tony Gutierrez <anthony.gutierrez@amd.com>

configs/common/CacheConfig.py

@@ -162,11 +162,6 @@ def config_cache(options, system):
         else:
             system.cpu[i].connectAllPorts(system.membus)
 
-        # Add a snoop filter to the membus if there are caches above it
-        if (options.l2cache or options.caches) and \
-        (system.membus.snoop_filter == NULL):
-            system.membus.snoop_filter = SnoopFilter()
-
     return system
 
 # ExternalSlave provides a "port", but when that port connects to a cache,