spidev: Enable the Liebherr's BK4 board to work with spidev driver

With this commit the DSPI driver on the BK4 board can be used for SPI
transmission managed from user space (via /dev/spidev0.0).

Example usage/testing:
insmod ./spi-fsl-dspi.ko
./spidev_test -D /dev/spidev0.0 -s 3000000 -v -H -b 8 -p "\xCC\x11\x22\x74"

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Mark Brown <broonie@kernel.org>