imx: add rules for U-Boot DTB support

Signed-off-by: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/config.mk b/arch/arm/config.mk
index 17b7408..5bd3961 100644
--- a/arch/arm/config.mk
+++ b/arch/arm/config.mk
@@ -119,6 +119,10 @@
 ALL-y += SPL
 endif
 else
+ifeq ($(CONFIG_OF_SEPARATE),y)
+ALL-y += u-boot-dtb.imx
+else
 ALL-y += u-boot.imx
 endif
 endif
+endif