spi: imx: Update device tree binding documentation

Update documentation for gpio-cs and num-cs to reflect the standard SPI
bindings.

The dma properties are optional.

Include a warning that native CS do not work in a commonly useful manner
with this hardware/driver, and therefor most users probably should use GPIO
based CS lines rather than native.

CC: Mark Brown <broonie@kernel.org>
CC: Shawn Guo <shawnguo@kernel.org>
CC: Sascha Hauer <kernel@pengutronix.de>
CC: Fabio Estevam <fabio.estevam@nxp.com>
CC: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Mark Brown <broonie@kernel.org>