config: Add a system clock command-line option

This patch adds a 'sys_clock' command-line option and use it to assign
clocks to the system during instantiation.

As part of this change, the default clock in the System class is
removed and whenever a system is instantiated a system clock value
must be set. A default value is provided for the command-line option.

The configs and tests are updated accordingly.

configs/splash2/cluster.py

@@ -214,6 +214,7 @@ else:
 system = System(cpu = all_cpus, l1_ = all_l1s, l1bus_ = all_l1buses,
                 physmem = SimpleMemory(),
                 membus = CoherentBus(clock = busFrequency))
+system.clock = '1GHz'
 
 system.toL2bus = CoherentBus(clock = busFrequency)
 system.l2 = L2(size = options.l2size, assoc = 8)

configs/splash2/run.py

@@ -199,6 +199,7 @@ else:
 # ----------------------
 system = System(cpu = cpus, physmem = SimpleMemory(),
                 membus = CoherentBus(clock = busFrequency))
+system.clock = '1GHz'
 
 system.toL2bus = CoherentBus(clock = busFrequency)
 system.l2 = L2(size = options.l2size, assoc = 8)