kbuild: make arch-dtbs target PHONY

Without this, the arch-dtbs target only gets evaluated when building
U-Boot the first time, not when re-building (incrementally building)
U-Boot. Thus incremental builds ignore changes to DTB files.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>

dts/Makefile

@@ -40,6 +40,7 @@ endif
 	echo >&2;							\
 	/bin/false)
 
+PHONY += arch-dtbs
 arch-dtbs:
 	$(Q)$(MAKE) $(build)=$(ARCH_PATH) dtbs