dt-bindings: kbuild: Support partial matches with DT_SCHEMA_FILES

DT_SCHEMA_FILES is currently restricted to a list of exact files with
the full source tree path (i.e. Documentation/devicetree/bindings/...).
Loosen this requirement and let DT_SCHEMA_FILES be a partial match.

With this, checking all schema files in a directory is possible:

$ make DT_SCHEMA_FILES=/gpio/ dt_binding_check

Or all schema files with 'qcom' in the path or filename:

$ make DT_SCHEMA_FILES=qcom dt_binding_check

Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20220228201006.1484903-1-robh@kernel.org