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>

src/mem/XBar.py

@@ -153,6 +153,9 @@ class SystemXBar(CoherentXBar):
     response_latency = 2
     snoop_response_latency = 4
 
+    # Use a snoop-filter by default
+    snoop_filter = SnoopFilter(lookup_latency = 1)
+
     # This specialisation of the coherent crossbar is to be considered
     # the point of coherency, as there are no (coherent) downstream
     # caches.