arm64: Fix the dtbs target building

The arch/arm64/Makefile was not passing the right target to the
boot/dts/Makefile.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

arch/arm64/Makefile

@@ -58,7 +58,7 @@ zinstall install: vmlinux
 	$(Q)$(MAKE) $(build)=$(boot)/dts $(boot)/dts/$@
 
 dtbs: scripts
-	$(Q)$(MAKE) $(build)=$(boot)/dts $(boot)/dts/$@
+	$(Q)$(MAKE) $(build)=$(boot)/dts dtbs
 
 # We use MRPROPER_FILES and CLEAN_FILES now
 archclean: