imx: add rules for U-Boot DTB support

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/imx-common/Makefile b/arch/arm/imx-common/Makefile
index 16809fe..025cfed 100644
--- a/arch/arm/imx-common/Makefile
+++ b/arch/arm/imx-common/Makefile
@@ -42,6 +42,14 @@
 u-boot.imx: u-boot.bin $(IMX_CONFIG) FORCE
 	$(call if_changed,mkimage)
 
+ifeq ($(CONFIG_OF_SEPARATE),y)
+MKIMAGEFLAGS_u-boot-dtb.imx = -n $(filter-out $< $(PHONY),$^) -T imximage \
+	-e $(CONFIG_SYS_TEXT_BASE)
+
+u-boot-dtb.imx: u-boot-dtb.bin $(IMX_CONFIG) FORCE
+	$(call if_changed,mkimage)
+endif
+
 MKIMAGEFLAGS_SPL = -n $(filter-out $< $(PHONY),$^) -T imximage \
 	-e $(CONFIG_SPL_TEXT_BASE)