kbuild: add dtbs_check to PHONY

The dtbs_check should be a phony target, but currently it is not
specified so.

'make dtbs_check' works even if a file named 'dtbs_check' exists
because it depends on another phony target, scripts_dtc, but we
should not rely on it.

Add dtbs_check to PHONY.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Rob 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 dt_binding_check
+PHONY += dtbs dtbs_install dtbs_check dt_binding_check
 dtbs dtbs_check: include/config/kernel.release scripts_dtc
 	$(Q)$(MAKE) $(build)=$(dtstree)