doc: mtk-wdt: support pre-timeout when the bark irq is available

Add description of pre-timeout in mtk-wdt.

Signed-off-by: Wang Qing <wangqing@vivo.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1619148020-2236-3-git-send-email-wangqing@vivo.com


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