dt-bindings: Re-enable core schemas for dtbs_check

In commit 2ba06cd ("kbuild: Always validate DT binding examples"),
the core schemas (from dtschema repo) were inadvertently disabled for
dtbs_checks. Re-enable them.

Fixes: 2ba06cd ("kbuild: Always validate DT binding examples")
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/Makefile

@@ -2,6 +2,7 @@
 DT_DOC_CHECKER ?= dt-doc-validate
 DT_EXTRACT_EX ?= dt-extract-example
 DT_MK_SCHEMA ?= dt-mk-schema
+DT_MK_SCHEMA_USERONLY_FLAG := $(if $(DT_SCHEMA_FILES), -u)
 
 quiet_cmd_chk_binding = CHKDT   $(patsubst $(srctree)/%,%,$<)
       cmd_chk_binding = $(DT_DOC_CHECKER) -u $(srctree)/$(src) $< ; \
@@ -37,7 +38,7 @@ override DTC_FLAGS := \
 $(obj)/processed-schema-examples.yaml: $(DT_DOCS) FORCE
 	$(call if_changed,mk_schema)
 
-$(obj)/processed-schema.yaml: DT_MK_SCHEMA_FLAGS := -u
+$(obj)/processed-schema.yaml: DT_MK_SCHEMA_FLAGS := $(DT_MK_SCHEMA_USERONLY_FLAG)
 $(obj)/processed-schema.yaml: $(DT_SCHEMA_FILES) FORCE
 	$(call if_changed,mk_schema)