mediatek: Separate WDT reset function from WDT driver

Separate WDT reset function from WDT driver, then we can use the common
WDT driver and have a board-specific reset function on different boards.

In Kukui, we plan to use GPIO HW reset, instead of WDT reset. Add config
"MISSING_BOARD_RESET" in Kukui to pass the build for now.

BUG=b:80501386
BRANCH=none
TEST=emerge-elm coreboot; emerge-kukui coreboot;

Change-Id: Ica07fe3a027cd7e9eb6d10202c3ef3ed7bea00c2
Signed-off-by: Tristan Shieh <tristan.shieh@mediatek.com>
Reviewed-on: https://review.coreboot.org/c/31121
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/mediatek/common/reset.c

@@ -0,0 +1,23 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright 2019 MediaTek Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#include <arch/io.h>
+#include <reset.h>
+#include <soc/wdt.h>
+
+void do_board_reset(void)
+{
+	write32(&mtk_wdt->wdt_swrst, MTK_WDT_SWRST_KEY);
+}