spi: spi-fsl-dspi: add SPI_LSB_FIRST to driver capabilities

The driver as well as the controller support the SPI lsb first
mode. However, it's not possible to configure it e.g. when using
spidev. Adding this flag to mode_bits resolves the issue and lsb first
mode can be used.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>