dt-bindings: pwm: Update bindings for MT7629 SoC

This updates bindings for MT7629 PWM controller.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>