mediatek: add support for Zbtlink ZBT-Z8102AX v2

mediatek: add support for Zbtlink ZBT-Z8102AX v2

Specifications:
SoC: MediaTek MT7981B
RAM: 1024MiB
Flash: SPI-NAND 128 MiB
Switch: 1 WAN, 4 LAN (Gigabit)
USB: two M.2 slots for 5G modems via USB 3.0 hub, external USB 3.0 port
Buttons: Reset, Mesh
Power: DC 12V 1A
WiFi: MT7976CN
UART: 115200n8
UART Layout:
VCC-RX-TX-GND

Installation:
1. Power down the router and hold in the Reset button.
2. While holding in the button power up the router again.
3. Hold the button in for 10 seconds and then release.
4. Use your browser to go to 192.168.1.1
5. If you see a GUI that is for flashing firmware then you have the V2 model.
	If there is no GUI and the router continues to boot up normally
	you have the V1 model.
6. Now use the V2 sysugrade file.

Note: Recovery GUI it can be used to recover from an incorrect firmware flash.

Based on patches adding support for this device by Yannick Chabanois (openmptcprouter)
and Dairyman (ofmodemsandmen)

Signed-off-by: Marius Durbaca <mariusd84@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18514
(cherry picked from commit c908fc7d9591163ae4836f31b7e6db4611e6bc7e)
Link: https://github.com/openwrt/openwrt/pull/19216
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>