ramips: add support for Zbtlink ZBT-WE2026

This patch adds support for the Zbtlink ZBT-WE2026.

Specification:
- SoC: MediaTek MT7620N (580MHz)
- RAM: 64 MiB
- Flash: 8 MiB SPI
- LAN: 4x100M
- WAN: 1x100M

Installation through bootloader webserver:
- With the power unplugged press and hold reset button.
- Plug power and hold reset button until LED starts to blink.
- Install sysupgrade image using web interface on 192.168.1.1.

Signed-off-by: Vaclav Svoboda <svoboda@neng.cz>