spi: dt-bindings: Introduce spi-cs-setup-ns property

SPI NOR flashes have specific cs-setup time requirements without which
they can't work at frequencies close to their maximum supported frequency,
as they miss the first bits of the instruction command. Unrecognized
commands are ignored, thus the flash will be unresponsive. Introduce the
spi-cs-setup-ns property to allow spi devices to specify their cs setup
time.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20221117105249.115649-2-tudor.ambarus@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>