dt-bindings: pwm: mediatek: Add documentation for MT8516

Add the device-tree documentation for the PWM IP on the MediaTek
MT8516 SoCs.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>