DT: watchdog: Add NXP LPC18xx Watchdog Timer binding documentation

* NXP LPC18xx Watchdog Timer (WDT)

Required properties:

- compatible: Should be "nxp,lpc1850-wwdt"

- reg: Should contain WDT registers location and length

- clocks: Must contain an entry for each entry in clock-names.

- clock-names: Should contain "wdtclk" and "reg"; the watchdog counter

               clock and register interface clock respectively.

- interrupts: Should contain WDT interrupt

Examples:

watchdog@40080000 {

	compatible = "nxp,lpc1850-wwdt";

	reg = <0x40080000 0x24>;

	clocks = <&cgu BASE_SAFE_CLK>, <&ccu1 CLK_CPU_WWDT>;

	clock-names = "wdtclk", "reg";

	interrupts = <49>;

};