dt-bindings: kbuild: Support single binding targets

dt-bindings: kbuild: Support single binding targets

Running the full 'make dt_binding_check' is slow. A shortcut is to set
DT_SCHEMA_FILES env variable to a substring of DT schema files to test.
It both limits which examples are validated and which schemas are used
to validate the examples. This is a problem because errors from other
schemas are missed. What makes validation slow is checking all examples,
so we really just need a way to test a single example.

Add a %.yaml target to validate the schema and validate the example:

make example-schema.yaml

The behavior for 'make dt_binding_check DT_SCHEMA_FILES=example-schema'
is unchanged. Really it should mirror dtbs_check and validate all the
examples with a subset of schemas, but there are lots of users of
expecting the existing behavior.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20251208224304.2907913-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>