spi: Add SPI driver for MT76xx SoCs

This patch adds the SPI driver for the MediaTek MT7688 SoC (and
derivates). Its been tested on the LinkIt Smart 7688 and the Gardena
Smart Gateway with and SPI NOR on CS0 and on the Gardena Smart
Gateway additionally with an SPI NAND on CS1.

Note that the SPI controller only supports a max transfer size of 32
bytes. This driver implementes a workaround to enable bigger xfer
sizes to speed up the transfer especially for the SPI NAND support.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Jagan Teki <jagan@openedev.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Piotr Dymacz <pepe2k@gmail.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>