pwm: Add NXP LPC18xx PWM/SCT DT binding documentation

Add the devicetree binding document for NXP LPC18xx PWM/SCT.
Signed-off-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/lpc1850-sct-pwm.txt

+* NXP LPC18xx State Configurable Timer - Pulse Width Modulator driver
+
+Required properties:
+  - compatible: Should be "nxp,lpc1850-sct-pwm"
+  - reg: Should contain physical base address and length of pwm registers.
+  - clocks: Must contain an entry for each entry in clock-names.
+    See ../clock/clock-bindings.txt for details.
+  - clock-names: Must include the following entries.
+    - pwm: PWM operating clock.
+  - #pwm-cells: Should be 3. See pwm.txt in this directory for the description
+    of the cells format.
+
+Example:
+  pwm: pwm@40000000 {
+    compatible = "nxp,lpc1850-sct-pwm";
+    reg = <0x40000000 0x1000>;
+    clocks =<&ccu1 CLK_CPU_SCT>;
+    clock-names = "pwm";
+    #pwm-cells = <3>;
+  };