watchdog MediaTek add upstream compatible

The upstream compatible is called mt6589-wdt.
Add this compatible to the driver.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
diff --git a/drivers/watchdog/mtk_wdt.c b/drivers/watchdog/mtk_wdt.c
index 669a323..b3c597e 100644
--- a/drivers/watchdog/mtk_wdt.c
+++ b/drivers/watchdog/mtk_wdt.c
@@ -143,6 +143,7 @@
 
 static const struct udevice_id mtk_wdt_ids[] = {
 	{ .compatible = "mediatek,wdt"},
+	{ .compatible = "mediatek,mt6589-wdt"},
 	{}
 };