Marvell PWM controller

Required properties:
- compatible: should be one or more of:
  - "marvell,pxa250-pwm"
  - "marvell,pxa270-pwm"
  - "marvell,pxa168-pwm"
  - "marvell,pxa910-pwm"
- reg: Physical base address and length of the registers used by the PWM channel
  Note that one device instance must be created for each PWM that is used, so the
  length covers only the register window for one PWM output, not that of the
  entire PWM controller.  Currently length is 0x10 for all supported devices.
- #pwm-cells: Should be 1.  This cell is used to specify the period in
  nanoseconds.

Example PWM device node:

pwm0: pwm@40b00000 {
	compatible = "marvell,pxa250-pwm";
	reg = <0x40b00000 0x10>;
	#pwm-cells = <1>;
};

Example PWM client node:

backlight {
	compatible = "pwm-backlight";
	pwms = <&pwm0 5000000>;
	...
}
