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>

Documentation/devicetree/bindings/pwm/pwm-mtk-disp.txt

@@ -6,6 +6,7 @@ Required properties:
    - "mediatek,mt6595-disp-pwm": found on mt6595 SoC.
    - "mediatek,mt8167-disp-pwm", "mediatek,mt8173-disp-pwm": found on mt8167 SoC.
    - "mediatek,mt8173-disp-pwm": found on mt8173 SoC.
+   - "mediatek,mt8183-disp-pwm": found on mt8183 SoC.$
  - reg: physical base address and length of the controller's registers.
  - #pwm-cells: must be 2. See pwm.yaml in this directory for a description of
    the cell format.