watchdog: mtk: add disable_wdt_extrst support

watchdog: mtk: add disable_wdt_extrst support

In some cases, we may need watchdog just to trigger an
internal soc reset without sending any output signal.

Provide a disable_wdt_extrst parameter for configuration.
We can disable or enable it just by configuring dts.

Signed-off-by: Fengquan Chen <fengquan.chen@mediatek.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210914123454.32603-3-Fengquan.Chen@mediatek.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>