dt-bindings: auxdisplay: img-ascii-lcd: Convert to json-schema

Convert the Device Tree binding documentation for ASCII LCD displays on
Imagination Technologies boards to json-schema.

Drop bogus regmap property.
Add example.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/6e74aa466d39ddc9abe502e054d04e8cc7b76b40.1627402094.git.geert@linux-m68k.org


[robh: add type to 'offset']
Signed-off-by: Rob Herring <robh@kernel.org>