dt-bindings: watchdog: mediatek: add support for MediaTek MT7623 and MT7622 SoC

This updates dt-binding documentation for MediaTek MT7622 and
MT7623 SoC. For the both SoCs supported all rely on the fallback
binding of the case with "mediatek,mt6589-wdt".

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
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>