dt-bindings: pwm: mediatek: Add documentation for MT8516

Add the device-tree documentation for the PWM IP on the MediaTek
MT8516 SoCs.
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/pwm-mediatek.txt

@@ -6,6 +6,7 @@ Required properties:
    - "mediatek,mt7622-pwm": found on mt7622 SoC.
    - "mediatek,mt7623-pwm": found on mt7623 SoC.
    - "mediatek,mt7628-pwm": found on mt7628 SoC.
+   - "mediatek,mt8516-pwm": found on mt8516 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.