checkpatch: Warn if DT bindings are not in schema format

DT bindings are moving to using a json-schema based schema format
instead of freeform text. Add a checkpatch.pl check to encourage using
the schema for new bindings. It's not yet a requirement, but is
progressively being required by some maintainers.

Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Rob Herring <robh@kernel.org>