MIPS: ralink: Add GARDENA smart Gateway MT7688 board

This patch adds support for the GARDENA smart Gateway, which is based on
the MediaTek MT7688 SoC. It is equipped with 128 MiB of DDR and 8 MiB of
flash (SPI NOR) and additional 128MiB SPI NAND storage.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Harvey Hunt <harveyhuntnexus@gmail.com>
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@vger.kernel.org