spi: lpspi: remove unused fsl_lpspi->chipselect

The cs-gpio is initailized by spi_get_gpio_descs() now. Remove the
chipselect.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Link: https://lore.kernel.org/r/20200727031448.31661-3-xiaoning.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>