TI SOC EHRPWM based PWM controller

Required properties:
- compatible: Must be "ti,<soc>-ehrpwm".
  for am33xx - compatible = "ti,am33xx-ehrpwm";
  for da850  - compatible = "ti,da850-ehrpwm", "ti,am33xx-ehrpwm";
- #pwm-cells: Should be 3. Number of cells being used to specify PWM property.
  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 in PWM specifier
  to 1 for inverse polarity & set to 0 for normal polarity.
- reg: physical base address and size of the registers map.

Optional properties:
- ti,hwmods: Name of the hwmod associated to the EHRPWM:
  "ehrpwm<x>", <x> being the 0-based instance number from the HW spec

Example:

ehrpwm0: ehrpwm@0 { /* EHRPWM on am33xx */
	compatible = "ti,am33xx-ehrpwm";
	#pwm-cells = <3>;
	reg = <0x48300200 0x100>;
	ti,hwmods = "ehrpwm0";
};

ehrpwm0: ehrpwm@0 { /* EHRPWM on da850 */
	compatible = "ti,da850-ehrpwm", "ti,am33xx-ehrpwm";
	#pwm-cells = <3>;
	reg = <0x300000 0x2000>;
};