soc/mediatek: Only update required bits when triggering WDT reset

To prevent to modify original value of wdt_mode, we use setbits32 to
update required bits.

BUG=b:218420108
TEST=build pass

Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Change-Id: I743c1af3583c18ec8500fc1eb89f31cdbce5317c
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61729
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/mediatek/common/wdt.c

@@ -32,7 +32,7 @@ int mtk_wdt_init(void)
 		 * console (either in SRAM or DRAM) can be flushed.
 		 */
 		dcache_clean_all();
-		write32(&mtk_wdt->wdt_mode, MTK_WDT_MODE_EXTEN | MTK_WDT_MODE_KEY);
+		setbits32(&mtk_wdt->wdt_mode, MTK_WDT_MODE_EXTEN | MTK_WDT_MODE_KEY);
 		write32(&mtk_wdt->wdt_swrst, MTK_WDT_SWRST_KEY);
 	} else if (wdt_sta & MTK_WDT_STA_SW_RST)
 		printk(BIOS_INFO, "normal software reboot\n");