The dt_binding_check is added to PHONY, but it is invisible when
$(dtstree) is empty. So, it is not specified as phony for
ARCH=x86 etc.

Add it to PHONY outside the ifneq ... endif block.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Acked-by: NRob Herring <robh@kernel.org>