mt8173: fix watchdog register setting

This patch corrects the watchdog register setting. To update watchdog
register, the watchdog mode key must be set to make the register
configurable.

Change-Id: I9ca98ea4012f7f220b116013461030de4638ce0b
Signed-off-by: Jimmy Huang <jimmy.huang@mediatek.com>

plat/mediatek/mt8173/plat_pm.c

@@ -497,8 +497,9 @@ static void __dead2 plat_system_reset(void)
 	/* Write the System Configuration Control Register */
 	INFO("MTK System Reset\n");
 
-	mmio_clrbits_32(MTK_WDT_BASE,
-		(MTK_WDT_MODE_DUAL_MODE | MTK_WDT_MODE_IRQ));
+	mmio_clrsetbits_32(MTK_WDT_BASE,
+		(MTK_WDT_MODE_DUAL_MODE | MTK_WDT_MODE_IRQ),
+		MTK_WDT_MODE_KEY);
 	mmio_setbits_32(MTK_WDT_BASE, (MTK_WDT_MODE_KEY | MTK_WDT_MODE_EXTEN));
 	mmio_setbits_32(MTK_WDT_SWRST, MTK_WDT_SWRST_KEY);