dt-bindings: Don't error out on yamllint and dt-doc-validate errors

A broken schema file now causes make to exit and 'make -k' no longer works
now that dt-doc-validate is called from a single make rule.

As yamllint is optional, we shouldn't stop on yamllint errors either. Also,
it seems some old versions of yamllint don't work.

Signed-off-by: Rob Herring <robh@kernel.org>