net: airoha: Introduce airoha_gdm_dev struct

net: airoha: Introduce airoha_gdm_dev struct

EN7581 and AN7583 SoCs support connecting multiple external SerDes to GDM3
or GDM4 ports via a hw arbiter that manages the traffic in a TDM manner.
As a result multiple net_devices can connect to the same GDM{3,4} port
and there is a theoretical "1:n" relation between GDM port and
net_devices.
Introduce airoha_gdm_dev struct to collect net_device related info (e.g.
net_device and external phy pointer). Please note this is just a
preliminary patch and we are still supporting a single net_device for
each GDM port. Subsequent patches will add support for multiple net_devices
connected to the same GDM port.

Tested-by: Xuegang Lu <xuegang.lu@airoha.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260527-airoha-eth-multi-serdes-preliminary-v1-1-ec6ed73ef7fc@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>