spi: fsl-dspi: add cs-gpios support

Make the driver be able to bit-bang a GPIO for the Chip Select pin of
select peripherals.

The GPIO value is driven by the driver in that case, and none of the
hardware Chip Select bits will be populated in the PUSHR register for
the TX commands constructed for this peripheral.

Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20221111211356.545026-1-vladimir.oltean@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>