dt-bindings: clk: Hi3660: Document stub clock

Document the DT binding for stub clock which is used for CPU,
GPU and DDR frequency scaling.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>