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>

drivers/watchdog/mtk_wdt.c

@@ -339,7 +339,7 @@ static int mtk_wdt_probe(struct platform_device *pdev)
 	if (IS_ERR(mtk_wdt->wdt_base))
 		return PTR_ERR(mtk_wdt->wdt_base);
 
-	irq = platform_get_irq(pdev, 0);
+	irq = platform_get_irq_optional(pdev, 0);
 	if (irq > 0) {
 		err = devm_request_irq(&pdev->dev, irq, mtk_wdt_isr, 0, "wdt_bark",
 				       &mtk_wdt->wdt_dev);