Introduce Airoha EN7581 entry in mediatek-gen3 PCIe controller binding.

Link: https://lore.kernel.org/linux-pci/138d65a140c3dcf2a6aefecc33ba6ba3ca300a23.1720022580.git.lorenzo@kernel.orgSigned-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Jianjun Wang <jianjun.wang@mediatek.com>