dt-bindings: watchdog: mediatek,mtk-wdt: Add compatible for MT8189 SoC

dt-bindings: watchdog: mediatek,mtk-wdt: Add compatible for MT8189 SoC

Add compatible string for the watchdog block on MT8189 SoC, which is
compatible with the one used on MT6589.

Signed-off-by: Louis-Alexis Eyraud <louisalexis.eyraud@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
pw-bot: not-applicable
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>