dt-bindings: serio: Convert Allwinner PS2 controller to a schema

The older Allwinner SoCs have a PS2 controller that 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 <mripard@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Rob Herring <robh@kernel.org>