mips: Add LinkIt Smart 7688 support

The LinkIt Smart 7688 modules have a MT7688 SoC with 128 MiB of RAM
and 32 MiB of flash (SPI NOR).

This patch also includes 2 targets. One is the target that can be
programmed into the SPI NOR flash and a 2nd target "xxx-ram" is
added to support loading and booting via an already running U-Boot
version. This allows easy development and testing without the
need to flash the image each time.

Signed-off-by: Stefan Roese <sr@denx.de>
[fixed and regenerated defconfig files]
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

board/seeed/linkit-smart-7688/Kconfig

@@ -0,0 +1,12 @@
+if BOARD_LINKIT_SMART_7688
+
+config SYS_BOARD
+	default "linkit-smart-7688"
+
+config SYS_VENDOR
+	default "seeed"
+
+config SYS_CONFIG_NAME
+	default "linkit-smart-7688"
+
+endif

board/seeed/linkit-smart-7688/MAINTAINERS

@@ -0,0 +1,8 @@
+LINKIT_SMART_7688 BOARD
+M:	Stefan Roese <sr@denx.de>
+S:	Maintained
+F:	board/seeed/linkit-smart-7688
+F:	include/configs/linkit-smart-7688.h
+F:	configs/linkit-smart-7688_defconfig
+F:	configs/linkit-smart-7688_ram_defconfig
+F:	arch/mips/dts/linkit-smart-7688.dts

board/seeed/linkit-smart-7688/Makefile

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0+
+
+obj-y += board.o

board/seeed/linkit-smart-7688/board.c

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2018 Stefan Roese <sr@denx.de>
+ */
+
+#include <common.h>
+#include <asm/io.h>
+
+#define MT76XX_GPIO1_MODE	0xb0000060
+
+void board_debug_uart_init(void)
+{
+	/* Select UART2 mode instead of GPIO mode (default) */
+	clrbits_le32((void __iomem *)MT76XX_GPIO1_MODE, GENMASK(27, 26));
+}
+
+int board_early_init_f(void)
+{
+	/*
+	 * The pin muxing of UART2 also needs to be done, if debug uart
+	 * is not enabled. So we need to call this function here as well.
+	 */
+	board_debug_uart_init();
+
+	return 0;
+}