pwm: Add NXP LPC18xx PWM/SCT DT binding documentation

* 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>;

  };