dt-bindings: net: dsa: mediatek,mt7530: fix reset lines

- Add description for reset-gpios.
- Invalidate reset-gpios if mediatek,mcm is used. We cannot use multiple
reset lines at the same time.
- Invalidate mediatek,mcm if the compatible device is mediatek,mt7531.
There is no multi-chip module version of mediatek,mt7531.
- Require mediatek,mcm for mediatek,mt7621 as the compatible string is only
used for the multi-chip module version of MT7530.

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