MIPS: dts: ralink: mt7628a: Update watchdog node according to bindings

MIPS: dts: ralink: mt7628a: Update watchdog node according to bindings

Most notably, add the mediatek,sysctl phandle and remove the redundant
reset/interrupt-related properties from the watchdog node. This is in
line with the corresponding devicetree (mt7628an.dtsi) used by the
OpenWrt project.

This has been tested on the MT7688-based GARDENA smart Gateway.

Signed-off-by: Ezra Buehler <ezra.buehler@husqvarnagroup.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>