mediatek: filogic: add support for Kebidumei AX3000-U22

mediatek: filogic: add support for Kebidumei AX3000-U22

Kebidumei AX3000-U22 is one of many clones of the same range extender
that can be found on Aliexpress or other Chinese portals.

The easiest way to identify this model is by searching for "AX3000
Repeater" and picking the device that looks like mine [0].

Specification:
- SoC:     MediaTek MT7981B (1.3 GHz)
- RAM:     256 MB
- Flash:   16 MB SPI NOR
- Ports:   1 x 1 GbE
- Antenna: 6 (2 fake)
- WiFi:    MediaTek dual-band WiFi 6
  - 2.4 GHz: b/g/n/ax, MIMO 2x2
  - 5 GHz:   a/n/ac/ax, MIMO 2x2
- Buttons: Reset & WPS
- LEDs:    Ethernet (green), Status (red, green, blue)
- Power:   110–240 V AC (internal PSU, board uses 12 V DC)
- Serial:  unmarked connector on PCB
           [1: Vcc, 2: RX, 3: TX, 4: GND]

Install via OEM web UI:
1. Use reset button to perform factory reset.
2. Connect PC to LAN port and obtain DHCP address.
3. Upload the sysupgrade image via OEM firmware upgrade page,
   e.g. http://192.168.18.1/upgrade.html
4. After reboot, hold reset button to clear leftover vendor config.

Install via serial:
1. Connect serial console (115200 8N1).
2. Enter the console.
3. Backup mtd4 partition if you want to restore OEM FW later.
4. Download image.
5. Run 'sysupgrade -n'.

Revert to stock:
1. Run sysupgrade without keeping config using mtd4 backup.

[0] https://openwrt.org/_media/media/kebidumei_ax3000-u22.png

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20287
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>