dt-bindings: pwm: Add MT2712/MT7622 information

Enhance the MediaTek PWM binding with details about the IP found in the
MT2712 and MT7622 SoCs.

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