dt-bindings: pwm: Add MT7628 information

Enhance the MediaTek PWM binding with details about the IP found in the
MT7628 SoC.
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

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

@@ -5,11 +5,13 @@ Required properties:
    - "mediatek,mt2712-pwm": found on mt2712 SoC.
    - "mediatek,mt7622-pwm": found on mt7622 SoC.
    - "mediatek,mt7623-pwm": found on mt7623 SoC.
+   - "mediatek,mt7628-pwm": found on mt7628 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:
+ - clock-names: must contain the following, except for MT7628 which
+                has no clocks
    - "top": the top clock generator
    - "main": clock used by the PWM core
    - "pwm1-8": the eight per PWM clocks for mt2712