dt-bindings: serial: mtk-uart: Convert txt to json-schema

Convert the mtk-uart documentation from freeform text to a json-schema.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220524115019.97246-1-angelogioacchino.delregno@collabora.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>