Since peripheral clocks are be derived in SoC or even platform
specific ways, we need a way to keep this information out of the
drivers.  AMBA Primecells are defined in terms of functional units
where clocks are derived from external sources, and it is up to the
SoC designer to determine where to derive those clocks from.

Therefore, we provide a very basic API which allows platforms to
provide this information in a generic manner to their peripherals.
This framework also allows peripherals to shut down unused clock
sources when they're not in use.