Generic GPIO bit-banged PWM driver

Required properties:
  - compatible: should be "pwm-gpio"
  - #pwm-cells: should be 3, see pwm.txt in this directory for a general
    description of the cells format.
  - pwm-gpio: one gpio describing the used gpio, see the gpio bindings for the used gpio driver.

Example:
#include <dt-bindings/gpio/gpio.h>

	pwm-gpio {
		compatible = "pwm-gpio";
		#pwm-cells = <3>;
		pwm-gpio = <&pio 7 1 GPIO_ACTIVE_HIGH>;
	};