spi: mcux: Rename spi driver to DSPI to match naming convention

In prep for supporting the older KL2x SoCs that use a different SPI
block, rename the current SPI driver to DSPI to match what the MCUX HAL
defines it as.

Change-Id: I9097580df5fca649ab6fd9a38212fced0b1ea6ed
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>