dt-bindings: pwm: Add MediaTek PWM bindings

Document the device-tree binding of MediaTek PWM. The PWM has 5 channels.
This has been tested on MT7623 only but should work on all the other MTK
SoCs that contain this core.

Signed-off-by: John Crispin <john@phrozen.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>