DT: watchdog: Add ImgTec PDC Watchdog Timer binding documentation

Add the devicetree binding document for ImgTec PDC Watchdog Timer.
Reviewed-by: NAndrew Bresticker <abrestic@chromium.org>
Signed-off-by: NNaidu Tellapati <Naidu.Tellapati@imgtec.com>
Signed-off-by: NJude Abraham <Jude.Abraham@gmail.com>
Signed-off-by: NEzequiel Garcia <ezequiel.garcia@imgtec.com>
Reviewed-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>

Documentation/devicetree/bindings/watchdog/imgpdc-wdt.txt

+*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>;
+};