dt-bindings: pwm: pwm-mediatek: Add documentation for MT6795 SoC

Add binding documentation for the MT6795 Helio X10 SoC.

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