imx8mm-cl-iot-gate: Remove redundant board_fix_fdt()

In arch/arm/mach-imx/imx8m/soc.c there's an implementation of
board_fix_fdt() introduced by commit 35bb6078. Remove the
redundant one to avoid failed to build from source when enabling
CONFIG_OF_BOARD_FIXUP.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: uboot-imx <uboot-imx@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>