dt-bindings: pwm: Add compatible for Mediatek MT8188

Add dt-binding documentation of pwm for MediaTek MT8188 SoC.

Signed-off-by: xinlei lee <xinlei.lee@mediatek.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>