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

The X-Powers AXP PMICs 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>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210924071614.868307-2-maxime@cerno.tech