dt-bindings: pwm: Convert pwm-mtk-disp.txt to mediatek,pwm-disp.yaml format

Convert pwm-mtk-disp.txt to mediatek,pwm-disp.yaml format as suggested
by maintainer.

Signed-off-by: Xinlei Lee <xinlei.lee@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>