kbuild: Always validate DT binding examples

Most folks only run dt_binding_check on the single schema they care about
by setting DT_SCHEMA_FILES. That means example is only checked against
that one schema which is not always sufficient.

Let's address this by splitting processed-schema.yaml into 2 files: one
that's always all schemas for the examples and one that's just the schema
in DT_SCHEMA_FILES for dtbs.

Co-developed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 752ff0a2..9754710 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -297,7 +297,8 @@
 
 DT_CHECKER ?= dt-validate
 DT_BINDING_DIR := Documentation/devicetree/bindings
-DT_TMP_SCHEMA := $(objtree)/$(DT_BINDING_DIR)/processed-schema.yaml
+# DT_TMP_SCHEMA may be overridden from Documentation/devicetree/bindings/Makefile
+DT_TMP_SCHEMA ?= $(objtree)/$(DT_BINDING_DIR)/processed-schema.yaml
 
 quiet_cmd_dtb_check =	CHECK   $@
       cmd_dtb_check =	$(DT_CHECKER) -u $(srctree)/$(DT_BINDING_DIR) -p $(DT_TMP_SCHEMA) $@