Add airoha_eth driver in order to introduce ethernet support for
Airoha EN7581 SoC available on EN7581 development board (en7581-evb).
EN7581 mac controller is mainly composed by the Frame Engine (PSE+PPE)
and QoS-DMA (QDMA) modules. FE is used for traffic offloading (just
basic functionalities are currently supported) while QDMA is used for
DMA operations and QOS functionalities between the mac layer and the
external modules conncted to the FE GDM ports (e.g MT7530 DSA switch
or external phys).
A general overview of airoha_eth architecture is reported below:

               ┌───────┐                                     ┌───────┐
               │ QDMA2 │                                     │ QDMA1 │
               └───┬───┘                                     └───┬───┘
                   │                                             │
           ┌───────▼─────────────────────────────────────────────▼────────┐
           │                                                              │
           │       P5                                            P0       │
           │                                                              │
           │                                                              │
           │                                                              │    ┌──────┐
           │                                                           P3 ├────► GDM3 │
           │                                                              │    └──────┘
           │                                                              │
           │                                                              │
┌─────┐    │                                                              │
│ PPE ◄────┤ P4                          PSE                              │
└─────┘    │                                                              │
           │                                                              │
           │                                                              │
           │                                                              │    ┌──────┐
           │                                                           P9 ├────► GDM4 │
           │                                                              │    └──────┘
           │                                                              │
           │                                                              │
           │                                                              │
           │        P2                                           P1       │
           └─────────┬───────────────────────────────────────────┬────────┘
                     │                                           │
                 ┌───▼──┐                                     ┌──▼───┐
                 │ GDM2 │                                     │ GDM1 │
                 └──────┘                                     └──┬───┘
                                                                 │
                                                            ┌────▼─────┐
                                                            │  MT7530  │
                                                            └──────────┘

Currently only hw LAN features (QDMA1+GDM1) are available while hw WAN
(QDMA2+GDM{2,3,4}) ones will be added with subsequent patches introducing
traffic offloading support.
Tested-by: Benjamin Larsson <benjamin.larsson@genexis.eu>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/274945d2391c195098ab180a46d0617b18b9e42c.1720818878.git.lorenzo@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>