spi: spi-fsl-dspi: DSPI support for NXP S32G

spi: spi-fsl-dspi: DSPI support for NXP S32G

Merge series from James Clark <james.clark@linaro.org>:

DT and driver changes for DSPI on S32G platforms. First 3 commits are
fixes for various edge cases which also apply to other platforms.
Remaining commits add new S32G registers and device settings, some S32G
specific fixes and then finally add the DT compatibles and binding docs.

Tested in both host and target mode on S32G-VNP-RDB3 by transferring to
an external device over spi1 using spidev_test.c