dt-bindings: Add a help message when dtschema tools are missing

The dtschema version check works, but is not that clear when dtschema is
either not installed or not in the PATH. Add a separate check and
message if dt-doc-validate is not found.

Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/Makefile

@@ -9,6 +9,11 @@ DT_SCHEMA_MIN_VERSION = 2021.2.1
 
 PHONY += check_dtschema_version
 check_dtschema_version:
+	@which $(DT_DOC_CHECKER) >/dev/null || \
+		{ echo "Error: '$(DT_DOC_CHECKER)' not found!" >&2; \
+		  echo "Ensure dtschema python package is installed and in your PATH." >&2; \
+		  echo "Current PATH is:" >&2; \
+		  echo "$$PATH" >&2; false; }
 	@{ echo $(DT_SCHEMA_MIN_VERSION); \
 	$(DT_DOC_CHECKER) --version 2>/dev/null || echo 0; } | sort -Vc >/dev/null || \
 	{ echo "ERROR: dtschema minimum version is v$(DT_SCHEMA_MIN_VERSION)" >&2; false; }