airoha: Introduce support multi-serdes on the same GDM port

airoha: Introduce support multi-serdes on the same GDM port

EN7581 or AN7583 SoCs support connecting multiple external SerDes (e.g.
Ethernet or USB 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 ports and net_devices.

           ┌─────────────────────────────────┐
           │                                 │    ┌──────┐
           │                         P1 GDM1 ├────►MT7530│
           │                                 │    └──────┘
           │                                 │      ETH0 (DSA conduit)
           │                                 │
           │              PSE/FE             │
           │                                 │
           │                                 │
           │                                 │    ┌─────┐
           │                         P0 CDM1 ├────►QDMA0│
           │  P4                     P9 GDM4 │    └─────┘
           └──┬─────────────────────────┬────┘
              │                         │
           ┌──▼──┐                 ┌────▼────┐
           │ PPE │                 │   ARB   │
           └─────┘                 └─┬─────┬─┘
                                     │     │
                                  ┌──▼──┐┌─▼───┐
                                  │ ETH ││ USB │
                                  └─────┘└─────┘
                                   ETH1   ETH2

This series introduces support for multiple net_devices connected to the
same Frame Engine (FE) GDM port (GDM3 or GDM4) via an external hw
arbiter. Please note GDM1 or GDM2 does not support the connection with
the external arbiter.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://github.com/openwrt/openwrt/pull/23481
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>