dt-bindings: Bump minimum version of dtschema to 2020.8.1

dtschema release 2020.8.1 gained several additions to help performance.
dt-doc-validate can now take a list of files and directories, and
dt-mk-schema can store the processed schema in JSON which is much faster
to parse than YAML. Utilizing both of these changes results in a 3-4x
speed improvement in running dt_binding_check.

There's also additional meta-schema checks which binding schemas should
be checked against.

Signed-off-by: Rob Herring <robh@kernel.org>