net: airoha: Enable support for multiple net_devices

net: airoha: Enable support for multiple net_devices

In the current codebase airoha_eth driver supports just a single
net_device connected to the Packet Switch Engine (PSE) lan port (GDM1).
As shown in commit 23020f049327 ("net: airoha: Introduce ethernet
support for EN7581 SoC"), PSE can switch packets between four GDM ports.
Enable the capability to create a net_device for each GDM port of the
PSE module. Moreover, since the QDMA blocks can be shared between
net_devices, do not stop TX/RX DMA in airoha_dev_stop() if there are
active net_devices for this QDMA block.
This is a preliminary patch to enable flowtable hw offloading for EN7581
SoC.

Co-developed-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>