ramips: mt76x8: Add support for Zbtlink ZBT-WE2426-B

ramips: mt76x8: Add support for Zbtlink ZBT-WE2426-B

The Zbtlink ZBT-WE2426-B is an indoor dual band WiFi router
with 4 external non detachable antennas and 5 Fast Ethernet ports.

Hardware of ZBT-WE2426-B:
- SoC: MT7628AN
- RAM: 64 MB (Winbond W9751G6K8-25)
- Storage: 8 MB SPI flash (S25FL064K)
- Ethernet: 5x 10/100 Mbps LAN1,LAN2,LAN3,LAN4 & WAN
- Wireless: 2.4GHz: on SoC (802.11b/g/n)
- Wireless: 5GHz: Mediatek MT7612EN (802.11n/ac)
- LEDs: 8x
- Buttons: 1x reset
- USB: 1x 2.0
- MicroSD slot: 1x
- Power: 9 VDC, 1 A
- Uart: GND TX RX PWR - J1 on the PCB
- Board silkscreen: "ZBT-WE2426-C V04" "2018-02-28" "CTT" "13 18"

Backup the stock firmware, settings and calibration data:
This router comes with PandoraBox OpenWrt firmware, so it is
possible to get all MTD partitions using scp.

Installation:
- Using the bootloader web server. Hold the reset button while turning
  the power on. Upload the sysupgrade image on http://192.168.1.1.
- Using the sysupgrade command in PandoraBox OpenWrt.

LEDs:
- LAN1,LAN2,LAN3,LAN4,WAN,WLAN2G use GPIO pins of the MT7628AN SoC
  (GPIOs 43,42,41,40,39,44)
- WLAN5G uses pin of MT7612EN.
- The POWER LED is directly connected to the VCC. It can be reconnected to
  the GPIO 37 of the MT7628AN SoC by resoldering SMD resistor on the PCB.

Buttons:
- The RESET button is connected to the GPIO 38 of the MT7628AN SoC.

MAC addresses as verified by OEM firmware:
use   address   source
2g    *:b0      factory 0x4 (label)
5g    *:b1      factory 0x8004
LAN   *:b2      factory 0x28
WAN   *:b3      factory 0x2e

Signed-off-by: Vaclav Svoboda <svoboda@neng.cz>
Link: https://github.com/openwrt/openwrt/pull/16927
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>