pwm: samsung: Update DT bindings documentation to cover clocks

PWM driver consumes at least one and up to three clocks, which need to be
specified in device tree when used. This patch updates bindings
documentation to add information about clocks.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

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

@@ -19,6 +19,16 @@ Required properties:
 - reg: base address and size of register area
 - interrupts: list of timer interrupts (one interrupt per timer, starting at
   timer 0)
+- clock-names: should contain all following required clock names:
+    - "timers" - PWM base clock used to generate PWM signals,
+  and any subset of following optional clock names:
+    - "pwm-tclk0" - first external PWM clock source,
+    - "pwm-tclk1" - second external PWM clock source.
+  Note that not all IP variants allow using all external clock sources.
+  Refer to SoC documentation to learn which clock source configurations
+  are available.
+- clocks: should contain clock specifiers of all clocks, which input names
+  have been specified in clock-names property, in same order.
 - #pwm-cells: number of cells used for PWM specifier - must be 3
    the specifier format is as follows:
      - phandle to PWM controller node
@@ -38,6 +48,8 @@ Example:
 		reg = <0x7f006000 0x1000>;
 		interrupt-parent = <&vic0>;
 		interrupts = <23>, <24>, <25>, <27>, <28>;
+		clocks = <&clock 67>;
+		clock-names = "timers";
 		samsung,pwm-outputs = <0>, <1>;
 		#pwm-cells = <3>;
 	}