dt-bindings: mfd: Convert Allwinner GPADC bindings to a schema

The Allwinner SoCs have an embedded GPADC that is doing thermal reading as
well, 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.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>