spi: spi-fsl-dspi: Update DT binding documentation

DSPI driver support the register on the controller with big endian
mode R/W. But not use the regmap API.
So update the binding documentation for "big-endian".

Signed-off-by: Yuan Yao <yao.yuan@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>