clk: qcom: clock-dummy: Add DT binding documentation

Qualcomm MSM Dummy Clock controller

Qualcomm MSM Dummy Clock controller devices provide a dummy clock for driver

development during pre-silicon stage. The driver will always return a dummy

clock that has no effect on hardware.

Required properties:

- compatible:		Must be "qcom,dummycc"

- #clock_cells:		Must be <1>. This will allow the common clock device

			tree framework to recognize _this_ device node as a

			clock provider.

Example:

	clock_rpm: qcom,rpmcc {

		compatible = "qcom,dummycc";

		#clock-cells = <1>;

	};