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