dt-bindings: watchdog: Add Nuvoton NPCM description

Nuvoton NPCM Watchdog

Nuvoton NPCM timer module provides five 24-bit timer counters, and a watchdog.

The watchdog supports a pre-timeout interrupt that fires 10ms before the

expiry.

Required properties:

- compatible      : "nuvoton,npcm750-wdt" for NPCM750 (Poleg).

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

- interrupts      : Contain the timer interrupt with flags for

                    falling edge.

Required clocking property, have to be one of:

- clocks          : phandle of timer reference clock.

- clock-frequency : The frequency in Hz of the clock that drives the NPCM7xx

                    timer (usually 25000000).

Optional properties:

- timeout-sec : Contains the watchdog timeout in seconds

Example:

timer@f000801c {

    compatible = "nuvoton,npcm750-wdt";

    interrupts = <GIC_SPI 47 IRQ_TYPE_LEVEL_HIGH>;

    reg = <0xf000801c 0x4>;

    clocks = <&clk NPCM7XX_CLK_TIMER>;

};