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>
diff --git a/board/compulab/imx8mm-cl-iot-gate/imximage-8mm-lpddr4.cfg b/board/compulab/imx8mm-cl-iot-gate/imximage-8mm-lpddr4.cfg
index b89092a..4071219 100644
--- a/board/compulab/imx8mm-cl-iot-gate/imximage-8mm-lpddr4.cfg
+++ b/board/compulab/imx8mm-cl-iot-gate/imximage-8mm-lpddr4.cfg
@@ -6,4 +6,4 @@
 #define __ASSEMBLY__
 
 BOOT_FROM	sd
-LOADER		mkimage.flash.mkimage	0x7E1000
+LOADER		u-boot-spl-ddr.bin	0x7e1000