NXP PCA9685 16-channel 12-bit PWM LED controller
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Required properties:
  - compatible: "nxp,pca9685-pwm"
  - #pwm-cells: should be 2. The first cell specifies the per-chip index
    of the PWM to use and the second cell is the period in nanoseconds.
    The index 16 is the ALLCALL channel, that sets all PWM channels at the same
    time.

Optional properties:
  - invert (bool): boolean to enable inverted logic
  - open-drain (bool): boolean to configure outputs with open-drain structure;
		       if omitted use totem-pole structure

Example:

For LEDs that are directly connected to the PCA, the following setting is
applicable:

pca: pca@41 {
	compatible = "nxp,pca9685-pwm";
	#pwm-cells = <2>;
	reg = <0x41>;
	invert;
	open-drain;
};
