PWM controlled by ChromeOS EC

Google's ChromeOS EC PWM is a simple PWM attached to the Embedded Controller
(EC) and controlled via a host-command interface.

An EC PWM node should be only found as a sub-node of the EC node (see
doc/device-tree-bindings/misc/cros-ec.txt).

Required properties:
- compatible: Must contain "google,cros-ec-pwm"
- #pwm-cells: Should be 1. The cell specifies the PWM index.

Example:
	cros-ec@0 {
		compatible = "google,cros-ec-spi";

		...

		cros_ec_pwm: ec-pwm {
			compatible = "google,cros-ec-pwm";
			#pwm-cells = <1>;
		};
	};