dt-bindings: Bump dtschema version required to v2021.2.1

There's several dependencies in dtschema since v2020.8.1 we need, so
let's bump the version required to v2021.2.1. Specifically, the
graph.yaml schema and improved undocumented compatible check are needed.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210311233640.1581526-1-robh@kernel.org