mediatek: add support Livinet Li320

mediatek: add support Livinet Li320

This commit adds support for Livinet Li320 wireless router.

Specification
-------------
- SoC       : MediaTek MT7981B dual-core ARM Cortex-A53
- RAM       : 512MB
- Flash     : SPI-NAND Fudan Micro FM25S01BI3
- WLAN      : MediaTek MT7976 dual-band Wi-Fi 6
- Ethernet  : MediaTek MT7531 switch, 1x WAN + 3x LAN (2.5GbE uplink via gmac0)
- Buttons   : WPS, Reset
- LEDs      : 8xGreen, 1xRed
- Power     : Barrel DC 12v

Installation
------------
The factory firmware is a compatible OpenWrt-based fork, allowing
direct installation via the LuCI upgrade page using the sysupgrade image.

MAC addresses
-------------
| Interface | MAC source               |
|-----------|--------------------------|
| LAN       | Factory 0x4 (mac-base -1)|
| WAN       | Factory 0x4 (mac-base -2)|
| WLAN 2.4G | Factory 0x4 (mac-base)   |
| WLAN 5G   | Factory 0x4 (mac-base +1)|

Signed-off-by: Fil Dunsky <filipp.dunsky@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23744
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>