watchdog: mtk_wdt: use platform_get_irq_optional

The watchdog pre-timeout (bark) interrupt is optional.  Use
platform_get_irq_optional() to avoid seeing such following
error message:
>>> mtk-wdt 10007000.watchdog: IRQ index 0 not found

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20211208095555.4099551-1-tzungbi@google.com


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