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>

Documentation/devicetree/bindings/spi/spi-fsl-dspi.txt

@@ -13,8 +13,7 @@ Required properties:
 
 Optional property:
 - big-endian: If present the dspi device's registers are implemented
-  in big endian mode, otherwise in native mode(same with CPU), for more
-  detail please see: Documentation/devicetree/bindings/regmap/regmap.txt.
+  in big endian mode.
 
 Optional SPI slave node properties:
 - fsl,spi-cs-sck-delay: a delay in nanoseconds between activating chip