dt-bindings: kbuild: Print a warning if yamllint is not found

Running yamllint is effectively required for binding schemas, so print a
warning if not found rather than silently skipping running it.
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20220303221417.2486268-1-robh@kernel.org

Documentation/devicetree/bindings/Makefile

@@ -3,7 +3,8 @@ DT_DOC_CHECKER ?= dt-doc-validate
 DT_EXTRACT_EX ?= dt-extract-example
 DT_MK_SCHEMA ?= dt-mk-schema
 
-DT_SCHEMA_LINT = $(shell which yamllint)
+DT_SCHEMA_LINT := $(shell which yamllint || \
+  echo "warning: python package 'yamllint' not installed, skipping" >&2)
 
 DT_SCHEMA_MIN_VERSION = 2021.2.1