arm64: dts: fsl-ls1028a: remove undocumented 'little-endian' for dspi node

arm64: dts: fsl-ls1028a: remove undocumented 'little-endian' for dspi node

According to binding doc http://devicetree.org/schemas/spi/fsl,dspi.yaml
and driver drivers/spi/spi-fsl-dspi.c, default is little-endian, should
use big-endian for big-endian system. Remove 'little-endian' to fix below
warning:

arch/arm64/boot/dts/freescale/fsl-ls1028a-qds-85bb.dtb: spi@2100000: Unevaluated properties are not allowed ('little-endian' was unexpected)

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>