board: mediatek: add MT7986 reference boards

Add general board files based on MT7986 SoCs.

MT7986 uses one mmc controller for booting from both SD and eMMC.
Both MT7986A and MT7986B use the same pins for spi controller.

Configs for various boot types:
1. mt7986_rfb_defconfig - SPI-NOR and SPI-NAND for MT7986A/B
2. mt7986a_bpir3_emmc_defconfig - eMMC for MT7986A only
3. mt7986a_bpir3_sd_defconfig - SD for MT7986A only

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

include/configs/mt7986.h

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Configuration for MediaTek MT7986 SoC
+ *
+ * Copyright (C) 2022 MediaTek Inc.
+ * Author: Sam Shih <sam.shih@mediatek.com>
+ */
+
+#ifndef __MT7986_H
+#define __MT7986_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