dt-bindings: auxdisplay: arm-charlcd: Convert to json-schema

Convert the ARM Versatile Character LCD Device Tree binding documentation
to json-schema.

Correct compatible value.
Document missing properties.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/4a63caa4136e8a31e82c7d75bb6f273498e8cccf.1627402256.git.geert@linux-m68k.org


Signed-off-by: Rob Herring <robh@kernel.org>