dt-bindings: net: dsa: mediatek,mt7530: fix description of mediatek,mcm

Fix the description of mediatek,mcm. mediatek,mcm is not used on MT7623NI.

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