net: airoha: Move airoha_eth driver in a dedicated folder

The airoha_eth driver has no codebase shared with mtk_eth_soc one.
Moreover, the upcoming features (flowtable hw offloading, PCS, ..) will
not reuse any code from MediaTek driver. Move the Airoha driver in a
dedicated folder.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>