clk: clock-dummy: Support parsing dummy_clocks from dt

Allow defining this clock type in dt. Dummy clocks do not
perform any writes to hw, and serve as a placeholder until a
real implementation is ready.

Change-Id: I3d54de7878e649a329ae312448b9f918fdcd2855
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>