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>