dt-binding: timer: document NPCM7xx timer DT bindings

Nuvoton NPCM7xx timer

Nuvoton NPCM7xx have three timer modules, each timer module provides five 24-bit

timer counters.

Required properties:

- compatible      : "nuvoton,npcm750-timer" for Poleg NPCM750.

- reg             : Offset and length of the register set for the device.

- interrupts      : Contain the timer interrupt with flags for

                    falling edge.

- clocks          : phandle of timer reference clock (usually a 25 MHz clock).

Example:

timer@f0008000 {

    compatible = "nuvoton,npcm750-timer";

    interrupts = <GIC_SPI 32 IRQ_TYPE_LEVEL_HIGH>;

    reg = <0xf0008000 0x50>;

    clocks = <&clk NPCM7XX_CLK_TIMER>;

};