Clock: Inherit the clock from parent by default

Clock: Inherit the clock from parent by default

This patch changes the default 1 Tick clock period to a proxy that
resolves the parents clock. As a result of this, the caches and
L1-to-L2 bus, for example, will automatically use the clock period of
the CPU unless explicitly overridden.

To ensure backwards compatibility, the System class overrides the
proxy and specifies a 1 Tick clock. We could change this to something
more reasonable in a follow-on patch, perhaps 1 GHz or something
similar.

With this patch applied, all clocked objects should have a reasonable
clock period set, and could start specifying delays in Cycles instead
of absolute time.