dt-bindings: opp: Convert Allwinner H6 OPP to a schema

The Allwinner H6 SoCs use binning in order to provide binning to
cpufreq which is supported in Linux, with a matching Device Tree binding.

Now that we have the DT validation in place, let's convert the device tree
bindings for that controller over to a YAML schemas.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Rob Herring <robh@kernel.org>