dt-bindings: net: dsa: mediatek,mt7530: Add airoha,en7581-switch

Add documentation for the built-in switch which can be found in the
Airoha EN7581 SoC.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>