dt: watchdog: Document compatibility with JZ4780

Suggested-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

Documentation/devicetree/bindings/watchdog/ingenic,jz4740-wdt.txt

-Ingenic Watchdog Timer (WDT) Controller for JZ4740
+Ingenic Watchdog Timer (WDT) Controller for JZ4740 & JZ4780
 
 Required properties:
-compatible: "ingenic,jz4740-watchdog"
+compatible: "ingenic,jz4740-watchdog" or "ingenic,jz4780-watchdog"
 reg: Register address and length for watchdog registers
 
 Example: