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