mediatek: add support for Routerich AX3000

mediatek: add support for Routerich AX3000

This PR is continuation of work under "mediatek: add support for Routerich
AX3000" #13703 by the agreement with PR #13703 original author (Maximilian
Weinmann <x1@disroot.org>). All reviews from the previous PR were taken
into into account.

Routerich AX3000 is a wireless WiFi 6 router.

Specification
-------------
- SoC       : MediaTek MT7981BA dual-core ARM Cortex-A53 1.3 GHz
- RAM       : DDR3 256 MiB (ESMT M15T2G16128A)
- Flash     : SPI-NAND 128 MiB (ESMT F50L1G41LB)
- WLAN      : MediaTek MT7976CN dual-band WiFi 6
  - 2.4 GHz : b/g/n/ax, MIMO 2x2
  - 5 GHz   : a/n/ac/ax, MIMO 2x2
- Ethernet  : 10/100/1000 Mbps x4 (MediaTek MT7531AE)
- USB       : 1x 2.0
- UART      : through-hole on PCB
  - [J500] GND, TX, RX, 3.3V (115200n8)
- Buttons   : Mesh, Reset
- LEDs      : 1x Power (Blue)
              1x WiFi 2.4 GHz (Blue)
              1x WiFi 5 GHz (Red)
              1x Mesh (Blue)
              3x LAN activity (Blue)
              1x WAN activity (Blue)
              2x WAN no-internet (Red)
- Power     : 12 VDC, 1.5 A

Installation
------------
Flash OpenWrt 'sysupgrade.bin' image using stock firmware web-interface
(without keeping settings).

Return to stock
---------------
Install stock firmware image (without keeping settings) using OpenWrt
sysupgrade method.

Recovery
--------
Connect uart, use u-boot menu to flash stock firmware image or boot
OpenWrt initramfs image.

MAC addresses
-------------
+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| WAN     | 24:0f:5e:xx:xx:b4 | label     |
| LAN     | 24:0f:5e:xx:xx:b5 | label+1   |
| WLAN 2g | 24:0f:5e:xx:xx:b6 | label+2   |
| WLAN 5g | 24:0f:5e:xx:xx:b7 | label+3   |
+---------+-------------------+-----------+
The WLAN 2g MAC was found in 'Factory', 0x4

Co-authored-by: Maximilian Weinmann <x1@disroot.org>
Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>