Documentation: Add device tree bindings for Samsung PWM timers

This patch adds missing documentation describing Device Tree bindings
for Samsung PWM timers.
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Olof Johansson <olof@lixom.net>

Documentation/devicetree/bindings/pwm/pwm-samsung.txt

+* 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>;
+	}