spi: imx: Implement support for CS_WORD

This only works when the native chipselect is in use. On a board with a
Ti ADS7950 8 channel ADC. This patch reduces the time to read out all
channels once from 280 us to 20 us.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210727124226.5571-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>