mem: Move crossbar default latencies to subclasses

This patch introduces a few subclasses to the CoherentXBar and
NoncoherentXBar to distinguish the different uses in the system. We
use the crossbar in a wide range of places: interfacing cores to the
L2, as a system interconnect, connecting I/O and peripherals,
etc. Needless to say, these crossbars have very different performance,
and the clock frequency alone is not enough to distinguish these
scenarios.

Instead of trying to capture every possible case, this patch
introduces dedicated subclasses for the three primary use-cases:
L2XBar, SystemXBar and IOXbar. More can be added if needed, and the
defaults can be overridden.

configs/common/CacheConfig.py

@@ -65,14 +65,12 @@ def config_cache(options, system):
     if options.l2cache:
         # Provide a clock for the L2 and the L1-to-L2 bus here as they
         # are not connected using addTwoLevelCacheHierarchy. Use the
-        # same clock as the CPUs, and set the L1-to-L2 bus width to 32
-        # bytes (256 bits).
+        # same clock as the CPUs.
         system.l2 = l2_cache_class(clk_domain=system.cpu_clk_domain,
                                    size=options.l2_size,
                                    assoc=options.l2_assoc)
 
-        system.tol2bus = CoherentXBar(clk_domain = system.cpu_clk_domain,
-                                      width = 32)
+        system.tol2bus = L2XBar(clk_domain = system.cpu_clk_domain)
         system.l2.cpu_side = system.tol2bus.master
         system.l2.mem_side = system.membus.slave