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>