dt-bindings: mfd: Convert X-Powers AC100 binding to a schema

The X-Powers AC100 hybrid devices are supported by Linux thanks to its
device tree binding.

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

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210924071614.868307-3-maxime@cerno.tech