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>

include/configs/linkit-smart-7688.h

@@ -0,0 +1,51 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (C) 2018 Stefan Roese <sr@denx.de>
+ */
+
+#ifndef __CONFIG_LINKIT_SMART_7688_H
+#define __CONFIG_LINKIT_SMART_7688_H
+
+/* CPU */
+#define CONFIG_SYS_MIPS_TIMER_FREQ	200000000
+
+/* RAM */
+#define CONFIG_SYS_SDRAM_BASE		0x80000000
+
+#define CONFIG_SYS_LOAD_ADDR		CONFIG_SYS_SDRAM_BASE + 0x100000
+
+#define CONFIG_SYS_INIT_SP_OFFSET	0x400000
+
+#ifdef CONFIG_BOOT_RAM
+#define CONFIG_SKIP_LOWLEVEL_INIT
+#endif
+
+/* UART */
+#define CONFIG_SYS_BAUDRATE_TABLE	{ 9600, 19200, 38400, 57600, 115200, \
+					  230400, 500000, 1500000 }
+
+/* RAM */
+#define CONFIG_SYS_MEMTEST_START	0x80100000
+#define CONFIG_SYS_MEMTEST_END		0x80400000
+
+/* Memory usage */
+#define CONFIG_SYS_MAXARGS		64
+#define CONFIG_SYS_MALLOC_LEN		(1024 * 1024)
+#define CONFIG_SYS_BOOTPARAMS_LEN	(128 * 1024)
+#define CONFIG_SYS_CBSIZE		512
+
+/* U-Boot */
+#define CONFIG_SYS_MONITOR_BASE		CONFIG_SYS_TEXT_BASE
+
+/* Environment settings */
+#define CONFIG_ENV_OFFSET		0x40000
+#define CONFIG_ENV_SIZE			(16 << 10)
+#define CONFIG_ENV_SECT_SIZE		(64 << 10)
+
+/*
+ * Environment is right behind U-Boot in flash. Make sure U-Boot
+ * doesn't grow into the environment area.
+ */
+#define CONFIG_BOARD_SIZE_LIMIT		CONFIG_ENV_OFFSET
+
+#endif /* __CONFIG_LINKIT_SMART_7688_H */