MLK-20060-6 dts: lpspi: Prevent CS line error in slave mode

MLK-20060-6 dts: lpspi: Prevent CS line error in slave mode

Prevent cs line configuration error in slave mode.

If master mode uses cs-gpio configuration, it should be changed into
non-gpio for CS line in slave mode. Otherwise, slave will receive several
0xFF at the beginning of one transfer especially in dma mode.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Acked-by: Fugang Duan <Fugang.duan@nxp.com>
Signed-off-by: Arulpandiyan Vadivel <arulpandiyan_vadivel@mentor.com>
Signed-off-by: Shrikant Bobade <Shrikant_Bobade@mentor.com>
(cherry picked from commit 88ba1025094d53fdecd1b443ef1a85514a132f21)