config: Remove redundant explicit setting of default clocks

This patch removes the explicit setting of the clock period for
certain instances of CoherentBus, NonCoherentBus and IOCache where the
specified clock is same as the default value of the system clock. As
all the values used are the defaults, there are no performance
changes. There are similar cases where the toL2Bus is set to use the
parent CPU clock which is already the default behaviour.

The main motivation for these simplifications is to ease the
introduction of clock domains.

src/cpu/BaseCPU.py

@@ -281,10 +281,10 @@ class BaseCPU(MemObject):
 
     def addTwoLevelCacheHierarchy(self, ic, dc, l2c, iwc = None, dwc = None):
         self.addPrivateSplitL1Caches(ic, dc, iwc, dwc)
-        # Override the default bus clock of 1 GHz and uses the CPU
-        # clock for the L1-to-L2 bus, and also set a width of 32 bytes
-        # (256-bits), which is four times that of the default bus.
-        self.toL2Bus = CoherentBus(clock = Parent.clock, width = 32)
+        # Set a width of 32 bytes (256-bits), which is four times that
+        # of the default bus. The clock of the CPU is inherited by
+        # default.
+        self.toL2Bus = CoherentBus(width = 32)
         self.connectCachedPorts(self.toL2Bus)
         self.l2cache = l2c
         self.toL2Bus.master = self.l2cache.cpu_side