ARM: dts: ls1021a: remove undocumented 'big-endian' for memory-controller node

ARM: dts: ls1021a: remove undocumented 'big-endian' for memory-controller node

According to binding doc memory-controllers/fsl/fsl,ddr.yaml
and driver drivers/edac/fsl_ddr_edac.c, default is big-endian, should
use little-endian for little-endian system. Remove 'big-endian' to fix
below warning:

arch/arm/boot/dts/nxp/ls/ls1021a-tqmls1021a-mbls1021a.dtb: memory-controller@1080000 (fsl,qoriq-memory-controller): 'big-endian' does not match any of the regexes: '^pinctrl-[0-9]+$'

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>