dt-bindings: net: airoha: Add EN7581 ethernet controller

Introduce device-tree binding documentation for Airoha EN7581 ethernet
mac controller.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/7dfecf8aa4e6519562a94455b95c49e1b3c858a0.1720818878.git.lorenzo@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>