Binding for an external clock signal driven by a PWM pin.

This binding uses the common clock binding[1] and the common PWM binding[2].

[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
[2] Documentation/devicetree/bindings/pwm/pwm.txt

Required properties:
- compatible : shall be "pwm-clock".
- #clock-cells : from common clock binding; shall be set to 0.
- pwms : from common PWM binding; this determines the clock frequency
  via the period given in the PWM specifier.

Optional properties:
- clock-output-names : From common clock binding.
- clock-frequency : Exact output frequency, in case the PWM period
  is not exact but was rounded to nanoseconds.

Example:
	clock {
		compatible = "pwm-clock";
		#clock-cells = <0>;
		clock-frequency = <25000000>;
		clock-output-names = "mipi_mclk";
		pwms = <&pwm2 0 40>; /* 1 / 40 ns = 25 MHz */
	};