dt-bindings: pwm: mediatek: Add mediatek,mt7986 compatible

Since commit 241eab76 ("pwm: mediatek: Add support for MT7986")
support for the 2 PWM channels implemented in MediaTek MT7986 SoCs has
been added. Also add the compatible string to dt-bindings now that
they have been converted to YAML.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>