mediatek: add support for Confiabits MT7981

mediatek: add support for Confiabits MT7981

Confiabits MT7981 is a Wi-Fi 6 router based on MediaTek MT7981.

Specification:
 - SoC: MediaTek MT7981B
 - CPU: 2x 1.3 GHz Cortex-A53
 - Flash: 128 MiB SPI NAND
 - RAM: 256 MiB
 - WLAN: 2.4 GHz, 5 GHz (MediaTek MT7976CN, 802.11ax)
 - Ethernet: 4x 10/100/1000 Mbps MT7531AE (3xLAN, 1xWAN)
 - USB 2.0 port
 - Buttons: 1 Reset button, 1 Mesh button.
 - LEDs: 7x light-blue, 2x warm-white
 - Serial console: internal 4-pin header, 115200 8n1
 - Power: 12 VDC, 1.5 A

MAC addresses in stock firmware and in this commit:
+---------+-------------------+-----------+
|         | MAC               | Algorithm |
+---------+-------------------+-----------+
| WAN     | 00:0c:43:xx:xx:e1 | label+1   |
| LAN     | 00:0c:43:xx:xx:e0 | label     |
| WLAN 2g | 00:0c:43:xx:xx:e0 | label     |
| WLAN 5g | 02:0c:43:xx:xx:e0 |           |
+---------+-------------------+-----------+
The label MAC was found in 'Factory', 0x4

Installation:
The stock firmware is OpenWrt-based. If you can reach LuCI or SSH, just use the sysupgrade image
with the 'Keep settings' option turned off.

Signed-off-by: Luis Mita <luis@luismita.com>