dt-bindings: watchdog: mtk-wdt: add disable_wdt_extrst support

This patch add a description and example of disable_wdt_extrst
element for watchdog on MTK Socs

Signed-off-by: Fengquan Chen <fengquan.chen@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210914123454.32603-2-Fengquan.Chen@mediatek.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>