Atmel TCB PWM controller

Required properties:
- compatible: should be "atmel,tcb-pwm"
- #pwm-cells: should be 3. See pwm.yaml in this directory for a description of
  the cells format. The only third cell flag supported by this binding is
  PWM_POLARITY_INVERTED.
- tc-block: The Timer Counter block to use as a PWM chip.

Example:

pwm {
	compatible = "atmel,tcb-pwm";
	#pwm-cells = <3>;
	tc-block = <1>;
};