kbuild: add dt_binding_check to PHONY in a correct place

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>

Makefile

@@ -1239,7 +1239,7 @@ ifneq ($(dtstree),)
 %.dtb: include/config/kernel.release scripts_dtc
 	$(Q)$(MAKE) $(build)=$(dtstree) $(dtstree)/$@
 
-PHONY += dtbs dtbs_install dtbs_check dt_binding_check
+PHONY += dtbs dtbs_install dtbs_check
 dtbs dtbs_check: include/config/kernel.release scripts_dtc
 	$(Q)$(MAKE) $(build)=$(dtstree)
 
@@ -1259,6 +1259,7 @@ PHONY += scripts_dtc
 scripts_dtc: scripts_basic
 	$(Q)$(MAKE) $(build)=scripts/dtc
 
+PHONY += dt_binding_check
 dt_binding_check: scripts_dtc
 	$(Q)$(MAKE) $(build)=Documentation/devicetree/bindings