spi: lpspi: fix using CS discontinuously on i.MX8DXLEVK

SPI common code does not support using CS discontinuously for now.
However, i.MX8DXL-EVK only uses CS1 without CS0. Therefore, add a flag
is_only_cs1 to set the correct TCR[PCS].

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Link: https://lore.kernel.org/r/20200727031448.31661-4-xiaoning.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>