imx8mm-cl-iot-gate: fix imximage intermediate binary naming

This fixes the following build time issue:

...
  BINMAN  all
binman: Error 1 running 'mkimage -d ./mkimage.spl.mkimage -n
 spl/u-boot-spl.cfgout -T imx8mimage -e 0x7e1000
 ./mkimage-out.spl.mkimage': mkimage.flash.mkimage: Can't open: No such
 file or directory

make: *** [Makefile:1094: all] Error 1

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>