board: mediatek: add MT7981 reference boards

This patch adds general board files based on MT7981 SoCs.

MT7981 uses one mmc controller for booting from both SD and eMMC, and the
pins of mmc controller are also shared with spi controller.
So three configs are need for these boot types:

1. mt7981_rfb_defconfig - SPI-NOR and SPI-NAND
2. mt7981_emmc_rfb_defconfig - eMMC only
3. mt7981_sd_rfb_defconfig - SD only

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>

include/configs/mt7981.h

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Configuration for MediaTek MT7981 SoC
+ *
+ * Copyright (C) 2022 MediaTek Inc.
+ * Author: Sam Shih <sam.shih@mediatek.com>
+ */
+
+#ifndef __MT7981_H
+#define __MT7981_H
+
+#include <linux/sizes.h>
+
+#define CONFIG_SYS_NONCACHED_MEMORY	SZ_1M
+
+/* Uboot definition */
+#define CONFIG_SYS_UBOOT_BASE		CONFIG_SYS_TEXT_BASE
+
+/* SPL -> Uboot */
+#define CONFIG_SYS_UBOOT_START		CONFIG_SYS_TEXT_BASE
+
+/* DRAM */
+#define CONFIG_SYS_SDRAM_BASE		0x40000000
+
+#endif