ramips: add support for ZBT-WE1326

Specification:
- SoC: MT7621AT, MT7603EN and MT7612EN
- Flash: 16 MiB (W25Q128FVSG)
- RAM: 512 MiB (EM6GE16EWXD-12H)
- Ethernet: 1 x WAN (10/100/1000Mbps) and 4 x LAN (10/100/1000 Mbps)
- Others: USB 2.0, micro SD slot, reset button and 8 x LEDs

Issues:
- Two LEDs for 2.4 GHz and 5 GHz Wi-Fi do not work, can't find GPIOs.
- The pwr LED is not GPIO controllable

How to install:
- The original firmware is OpenWrt, so both LuCI or sysupgrade can be used.
- Do not keep settings, for sysupgrade please use the -n option.

Signed-off-by: Jiawei Wang <buaawjw@gmail.com>