configs: imx8mm-cl-iot-gate: update dfu_alt_info for single flash.bin

We changed to single flash.bin now. So dfu_alt_info should be modified
to reflect this change.

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