dt-bindings: clock: Add DT bindings for Actions Semi S500 CMU

Add devicetree bindings for Actions Semi S500 Clock Management Unit.

Signed-off-by: Edgar Bernardi Righi <edgar.righi@lsitec.org.br>
[Mani: Documented S500 CMU compatible]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
[sboyd@kernel.org: Fix SPDX comment style in header file]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>