dt-bindings: net: dsa: mediatek,mt7530: make trivial changes

Make trivial changes on the binding.

- Update title to include MT7531 switch.
- Add me as a maintainer. List maintainers in alphabetical order by first
name.
- Add description to compatible strings.
- Stretch descriptions up to the 80 character limit.
- Remove lists for single items.
- Remove requiring reg as it's already required by dsa-port.yaml.
- Define acceptable reg values for the CPU ports.
- Remove quotes from $ref: "dsa.yaml#".

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>