ramips: add support for Zbtlink ZBT-WG108

ramips: add support for Zbtlink ZBT-WG108

Specification:

CPU: MediaTek MT7621 (880 MHz)
Flash size: 16 MB NOR SPI
RAM size: 128 MB
Bootloader: Breed
Wireless: MT7612EN 2x2 802.11an+ac(5 GHz)
Wireless: MT7603EN 2x2 bgn(2.4 GHz)
Ethernet: 1 x WAN (10/100/1000Mbps) and 4 x LAN (10/100/1000 Mbps)
USB: 1x 2.0 Type-A port
External storage: 1x microSD (SDXC) slot
UART: console (115200 baud)
LEDs: Power, Wan, Lan 1-4, WiFi 2.4G, WiFi 5G
Buttons: Reset

How to install:
The original firmware is OpenWrt, so sysupgrade can be used.

Signed-off-by: Nickolay Goppen <setotau@mainlining.org>
Link: https://github.com/openwrt/openwrt/pull/19966
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>