dt-bindings: Check the examples against the schemas

Currently, the binding examples are just built with dtc. dtc recently
gained the support necessary to output the examples in YAML format
(commit 87963ee20693 ("livetree: add missing type markers in generated
overlay properties"). Now just switch the output format and the examples
will be checked against the schema.

Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>