drivers: spi_mcux_lpspi: Fix DMA path on RT

Apparently, the previous change to fix the chip select only works on
some newer revisions of the LPSPI, and the behavior is different on
older parts like RT. For now put some #ifdef to keep the fixed CS on
MCXN but have the old behavior on RT and other platforms.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>