DT: watchdog: Add ImgTec PDC Watchdog Timer binding documentation

*ImgTec PowerDown Controller (PDC) Watchdog Timer (WDT)

Required properties:

- compatible : Should be "img,pdc-wdt"

- reg : Should contain WDT registers location and length

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

- clock-names: Should contain "wdt" and "sys"; the watchdog counter

               clock and register interface clock respectively.

- interrupts : Should contain WDT interrupt

Examples:

watchdog@18102100 {

	compatible = "img,pdc-wdt";

	reg = <0x18102100 0x100>;

	clocks = <&pdc_wdt_clk>, <&sys_clk>;

	clock-names = "wdt", "sys";

	interrupts = <0 52 IRQ_TYPE_LEVEL_HIGH>;

};