mediatek: add support for ipTIME AX3000SM

mediatek: add support for ipTIME AX3000SM

Specification
-------------
- SoC       : MediaTek MT7981BA dual-core ARM Cortex-A53 1.3GHz
- RAM       : DDR3 256Mbytes, Nanya Technology NT5CC128M16IP
- Flash     : 128Mbytes NAND Flash, ESMT F50L1G41LB
- WLAN      : MediaTek MT7976CN dual-band Wi-Fi 6
  - 2.4GHz  : b/g/n/ax, MU-MIMO
  - 5GHz    : a/n/ac/ax, MU-MIMO
- Ethernet  : 10/100/1000 Mbps x4, LAN (MediaTek MT7531AE)
              10/100/1000 Mbps x1, WAN (MT7981 internal PHY)
- UART      : 1x4 pin header on PCB
  - [J500] 3.3V, TX, RX, GND (115200, 8N1)
- Buttons   : WPS, Reset
- LEDs      : 1x Power (Amber)
              1x CPU (Amber)
              1x Wi-Fi 5GHz (Amber)
              1x Wi-Fi 2.4GHz (Amber)
              1x WAN activity (Amber)
              4x LAN activity (Amber)
- Power     : 12VDC, 1A (Center positive polarity)

MAC address
-----------
+-----------+-------------------+-----------+
| Interface | MAC               | Algorithm |
+-----------+-------------------+-----------+
| WLAN 2.4G | B0:38:6C:xx:xx:xx | label     |
| WLAN 5G   | B2:38:6C:4x:xx:xx |           |
| WAN       | B0:38:6C:xx:xx:xx | label + 1 |
| LAN       | B0:38:6C:xx:xx:xx | label + 3 |
+-----------+-------------------+-----------+
The WLAN 2.4G MAC address was found in 'Factory' partition, 0x4

Installation
------------
1. Download the *initramfs-kernel.bin file from the OpenWrt website
2. Attach UART to the router, and interrupt the boot process by pressing '0'

   If you successfully interrupt the boot process, a terminal prompt
   name should look like this:

   MT7981>

3. Connect the router(LAN port) to the PC
4. Assign the PC IP address: 192.168.0.100/24
5. Load and run the *initramfs-kernel.bin:

  tftpboot 0x46000000 initramfs-kernel.bin
  bootm

6. Upload the OpenWrt *squashfs-sysupgrade.bin to the router
7. Run 'sysupgrade -n' with the sysupgrade OpenWrt image

Signed-off-by: Donghyun Ko <nyankosoftware@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18689
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>