dt-bindings: pwm: mtk-disp: Add compatible string for MT8183 SoC

Add compatible string for MT8183 SoC in device tree binding.

Signed-off-by: Allen-KH Cheng <Allen-KH.Cheng@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>