spi/mxs: Add SSP/SPI device tree documentation

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Chris Ball <cjb@laptop.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>