dt-bindings: lpspi: New property in document DT bindings for LPSPI

Add "fsl,spi-only-use-cs1-sel" to fit i.MX8DXL-EVK.
Spi common code does not support use of CS signals discontinuously.
It only uses CS1 without using CS0. So, add this property to re-config
chipselect value.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Link: https://lore.kernel.org/r/20200727031513.31774-1-xiaoning.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>