Create private sysctl nodes for inspecting clock trees.

As discussed on tech-kern@ 10 days ago, add a clk_round_rate() method,
which returns the rate that would be used by this clock if clk_set_rate()
was called. Used by drivers (or other clocks) which ha

As discussed on tech-kern@ 10 days ago, add a clk_round_rate() method,
which returns the rate that would be used by this clock if clk_set_rate()
was called. Used by drivers (or other clocks) which have their own divider
and need to know the parent's clock capabilities to compute the best
parameters.

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Add support for multiple clock domains in clk API.

Add basic clock device infrastructure.