net: airoha: Introduce WAN device flag

net: airoha: Introduce WAN device flag

Introduce WAN flag to specify if a given device is used to transmit/receive
WAN or LAN traffic. Current codebase supports specifying LAN/WAN device
configuration in ndo_init() callback during device bootstrap.
In order to consider setups where LAN configuration is used even for
GDM3/GDM4 devices, check airoha_is_lan_gdm_dev() to select pse_port in
airoha_ppe_foe_entry_prepare().
Please note after this patch, it will be possible to specify multiple LAN
devices but just a single WAN one. Please note this change is not visible
to the user since airoha_eth driver currently supports just the internal
phy available via the MT7530 DSA switch and there are no WAN interfaces
officially supported since PCS/external phy is not merged mainline yet
(it will be posted with following patches).

Tested-by: Xuegang Lu <xuegang.lu@airoha.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260603-airoha-eth-multi-serdes-v9-5-5d476bc2f426@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>