dt-bindings: pwm: mediatek: Convert pwm-mediatek to DT schema

This converts pwm-mediatek.txt to mediatek,mt2712-pwm.yaml schema;
while at it, the clock names were clarified as previously they were
documented as "pwmX-Y", but valid names are "pwmN" only.
Also, the example was changed to use "mediatek,mt2712-pwm" instead
for consistency with the schema filename.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>