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>