imx8mn_evk: Add the missing spl.bin entry

The generated flash.bin does not boot the imx8mn evk LPDDR4 variant
as it misses the spl.bin description in binman.

Add its entry to fix the boot on the imx8mn evk LPDDR4 variant.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Tested-by: Arti Zirk <art@zirk.me>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>