arm, imx, Makefile: fix u-boot-dtb.imx build in CONFIG_MULTI_DTB_FIT case

in case CONFIG_MULTI_DTB_FIT is set and u-boot-dtb.imx
image is build, currently u-boot-dtb.bin is used for
generating the u-boot-dtb.imx binary, which is wrong, as
it contains only a dtb blob not the fit.blob

Use instead the u-boot-fit-dtb.bin for generating
u-boot-dtb.imx which contains the fit.blob.

Signed-off-by: Heiko Schocher <hs@denx.de>
diff --git a/Makefile b/Makefile
index 8de3d41..7e5fd23 100644
--- a/Makefile
+++ b/Makefile
@@ -1087,6 +1087,10 @@
 
 u-boot.bin: u-boot-fit-dtb.bin FORCE
 	$(call if_changed,copy)
+
+u-boot-dtb.bin: u-boot-nodtb.bin dts/dt.dtb FORCE
+	$(call if_changed,cat)
+
 else ifeq ($(CONFIG_OF_SEPARATE),y)
 u-boot-dtb.bin: u-boot-nodtb.bin dts/dt.dtb FORCE
 	$(call if_changed,cat)