Documentation: Add device tree bindings for Samsung PWM timers

* Samsung PWM timers

Samsung SoCs contain PWM timer blocks which can be used for system clock source

and clock event timers, as well as to drive SoC outputs with PWM signal. Each

PWM timer block provides 5 PWM channels (not all of them can drive physical

outputs - see SoC and board manual).

Be aware that the clocksource driver supports only uniprocessor systems.

Required properties:

- compatible : should be one of following:

    samsung,s3c2410-pwm - for 16-bit timers present on S3C24xx SoCs

    samsung,s3c6400-pwm - for 32-bit timers present on S3C64xx SoCs

    samsung,s5p6440-pwm - for 32-bit timers present on S5P64x0 SoCs

    samsung,s5pc100-pwm - for 32-bit timers present on S5PC100, S5PV210,

			  Exynos4210 rev0 SoCs

    samsung,exynos4210-pwm - for 32-bit timers present on Exynos4210,

                          Exynos4x12 and Exynos5250 SoCs

- reg: base address and size of register area

- interrupts: list of timer interrupts (one interrupt per timer, starting at

  timer 0)

- #pwm-cells: number of cells used for PWM specifier - must be 3

   the specifier format is as follows:

     - phandle to PWM controller node

     - index of PWM channel (from 0 to 4)

     - PWM signal period in nanoseconds

     - bitmask of optional PWM flags:

        0x1 - invert PWM signal

Optional properties:

- samsung,pwm-outputs: list of PWM channels used as PWM outputs on particular

    platform - an array of up to 5 elements being indices of PWM channels

    (from 0 to 4), the order does not matter.

Example:

	pwm@7f006000 {

		compatible = "samsung,s3c6400-pwm";

		reg = <0x7f006000 0x1000>;

		interrupt-parent = <&vic0>;

		interrupts = <23>, <24>, <25>, <27>, <28>;

		samsung,pwm-outputs = <0>, <1>;

		#pwm-cells = <3>;

	}