imx8mn-var-som: fix m7 binary loading when using the TCM

Starting from commit 9cc2323, is no
longer possible loading binaries directly to TCM.
Let's copy in DDR and then to the final target address.

include/configs/imx8mn_var_som.h

@@ -127,7 +127,8 @@
 	"m7_addr=0x7e0000\0" \
 	"m7_bin=hello_world.bin\0" \
 	"use_m7=no\0" \
-	"loadm7bin=load mmc ${mmcdev}:${mmcpart} ${m7_addr} ${bootdir}/${m7_bin}\0" \
+	"loadm7bin=load mmc ${mmcdev}:${mmcpart} ${loadaddr} ${bootdir}/${m7_bin}; " \
+		"cp.b ${loadaddr} ${m7_addr} ${filesize}\0" \
 	"runm7bin=" \
 		"if test ${m7_addr} = 0x7e0000; then " \
 			"echo Booting M7 from TCM; " \