TI SOC based PWM Subsystem

Required properties:
- compatible: Must be "ti,<soc>-pwmss".
  for am33xx  - compatible = "ti,am33xx-pwmss";
  for am4372  - compatible = "ti,am4372-pwmss","ti,am33xx-pwmss";
  for dra746 - compatible = "ti,dra746-pwmss", "ti,am33xx-pwmss"

- reg: physical base address and size of the registers map.
- address-cells: Specify the number of u32 entries needed in child nodes.
		  Should set to 1.
- size-cells: specify number of u32 entries needed to specify child nodes size
		in reg property. Should set to 1.
- ranges: describes the address mapping of a memory-mapped bus. Should set to
	   physical address map of child's base address, physical address within
	   parent's address  space and length of the address map. For am33xx,
	   3 set of child register maps present, ECAP register space, EQEP
	   register space, EHRPWM register space.

Also child nodes should also populated under PWMSS DT node.

Example:
epwmss0: epwmss@48300000 { /* PWMSS for am33xx */
	compatible = "ti,am33xx-pwmss";
	reg = <0x48300000 0x10>;
	ti,hwmods = "epwmss0";
	#address-cells = <1>;
	#size-cells = <1>;
	status = "disabled";
	ranges = <0x48300100 0x48300100 0x80   /* ECAP */
		  0x48300180 0x48300180 0x80   /* EQEP */
		  0x48300200 0x48300200 0x80>; /* EHRPWM */

	/* child nodes go here */
};

epwmss0: epwmss@48300000 { /* PWMSS for am4372 */
	compatible = "ti,am4372-pwmss","ti,am33xx-pwmss"
	reg = <0x48300000 0x10>;
	ti,hwmods = "epwmss0";
	#address-cells = <1>;
	#size-cells = <1>;
	status = "disabled";
	ranges = <0x48300100 0x48300100 0x80   /* ECAP */
		  0x48300180 0x48300180 0x80   /* EQEP */
		  0x48300200 0x48300200 0x80>; /* EHRPWM */

	/* child nodes go here */
};

epwmss0: epwmss@4843e000 { /* PWMSS for DRA7xx */
	compatible = "ti,dra746-pwmss", "ti,am33xx-pwmss";
	reg = <0x4843e000 0x30>;
	ti,hwmods = "epwmss0";
	#address-cells = <1>;
	#size-cells = <1>;
	ranges;

	/* child nodes go here */
};