mips: dts: ralink: mt7621: add phandle to system controller node for watchdog

To allow to access system controller registers from watchdog driver code
add a phandle in the watchdog 'wdt' node. This avoid using arch dependent
operations in driver code.
Reviewed-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/ralink/mt7621.dtsi

@@ -73,6 +73,7 @@ sysc: syscon@0 {
 		wdt: watchdog@100 {
 			compatible = "mediatek,mt7621-wdt";
 			reg = <0x100 0x100>;
+			mediatek,sysctl = <&sysc>;
 		};
 
 		gpio: gpio@600 {