Document the device-tree binding of MediatTek display PWM.
The PWM has one channel to control the backlight brightness for display.
It supports MT8173 and MT6595.

Signed-off-by: YH Huang <redacted>
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 .../devicetree/bindings/pwm/pwm-mtk-disp.txt       | 24 ++++++++++++++++++++++
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 create mode 100644 Documentation/devicetree/bindings/pwm/pwm-mtk-disp.txt

diff --git a/Documentation/devicetree/bindings/pwm/pwm-mtk-disp.txt b/Documentation/devicetree/bindings/pwm/pwm-mtk-disp.txt
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+MediaTek display PWM controller
+
+Required properties:
+ - compatible: should be "mediatek,<name>-disp-pwm"
+   - "mediatek,mt8173-disp-pwm": found on mt8173 SoC
+   - "mediatek,mt6595-disp-pwm": found on mt6595 SoC
+ - reg: physical base address and length of the controller's registers
+ - #pwm-cells: must be 2. See pwm.txt in this directory for a description of
+   the cell format
+ - clocks: phandle and clock specifier of the PWM reference clock
+ - clock-names: must contain the following
+   - "main": clock used to generate PWM signals
+   - "mm": sync signals from the modules of mmsys
+
+Example:
+       pwm0: pwm@1401e000 {
+               compatible = "mediatek,mt8173-disp-pwm",
+                            "mediatek,mt6595-disp-pwm";
+               reg = <0 0x1401e000 0 0x1000>;
+               #pwm-cells = <2>;
+               clocks = <&mmsys CLK_MM_DISP_PWM026M>,
+                        <&mmsys CLK_MM_DISP_PWM0MM>;
+               clock-names = "main", "mm";