Atmel TCB PWM controller

Required properties:
- compatible: should be "atmel,tcb-pwm"
- #pwm-cells: Should be 3.  The first cell specifies the per-chip index
  of the PWM to use, the second cell is the period in nanoseconds and
  bit 0 in the third cell is used to encode the polarity of PWM output.
  Set bit 0 of the third cell in PWM specifier to 1 for inverse polarity &
  set to 0 for normal polarity.
- tc-block: The Timer Counter block to use as a PWM chip.

Example:

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