mediatek: add support for Zbtlink ZBT-Z8103AX

mediatek: add support for Zbtlink ZBT-Z8103AX

Specifications:
SoC: MediaTek MT7981B
RAM: 256MiB
Flash: SPI-NAND 128 MiB
Switch: 1 WAN, 3 LAN (Gigabit)
Buttons: Reset, Mesh
Power: DC 12V 1A
WiFi: MT7976CN
UART: 115200n8
UART Layout:
VCC-RX-TX-GND

No. of Antennas: 6
Note: Upon opening the router, only 5 antennas were connected
to the mainboard.

Led Layout:
Power-Mesh-5gwifi-WAN-LAN3-LAN2-LAN1-2gWiFi

Buttons:
Reset-Mesh

Installation:
A. Through OpenWrt Dashboard:
If your router comes with OpenWrt preinstalled (modified by the seller),
you can easily upgrade by going to the dashboard (192.168.1.1) and then
navigate to System -> Backup/Flash firmware, then flash the firmware

B. Through TFTP
Standard installation via UART:

1. Connect USB Serial Adapter to the UART, (NOTE: Don't connect the VCC pin).
2. Power on the router. Make sure that you can access your router via UART.
3. Restart the router then repeatedly press ctrl + c to skip default boot.
4. Type > bootmenu
5. Press '2' to select upgrade firmware
6. Press 'Y' on 'Run image after upgrading?'
7. Press '0' and hit 'enter' to select TFTP client (default)
8. Fill the U-Boot's IP address and TFTP server's IP address.
9. Finally, enter the 'firmware' filename.

Signed-off-by: Ian Oderon <ianoderon@gmail.com>