ramips: mt7621: add support for Zbtlink ZBT-WG1608

Zbtlink ZBT-WG1608 is a Wi-Fi router intendent to use with WWAN (4G/5G)
modems.

Specifications:
* SoC: MediaTek MT7621A
* RAM: 256/512 MiB
* Flash: 16/32 MiB (SPI NOR)
* Wi-Fi:
  * MediaTek MT7603E : 2.4Ghz
  * MediaTek MT7613BE : 5Ghz
* Ethernet: 10/100/1000 Mbps Ethernet x5 ports (4xLAN + WAN)
* M.2: 1x slot with USB&SIM
  * EM7455/EM12-G/EM160R/RM500Q-AE
* USB: 1x 3.0 Type-A port
* External storage: 1x microSD (SDXC) slot
* UART: console (115200 baud)
* LED:
  * 1 power indicator
  * 1 WLAN 2.4G controlled (wlan 2G)
  * 3 SoC controlled (wlan 5G, wwan, internet)
  * 5 per Eth phy (4xLAN + WAN)

MAC Addresses:
* LAN    : f8:5e:3c:xx:xx:e0 (Factory, 0xe000 (hex))
* WAN    : f8:5e:3c:xx:xx:e1 (Factory, 0xe006 (hex))
* 2.4 GHz: f8:5e:3c:xx:xx:de (Factory, 0x0004 (hex))
* 5 GHz  : f8:5e:3c:xx:xx:df (Factory, 0x8004 (hex))

Installation:
* Vendor's firmware is OpenWrt (LEDE) based, so the sysupgrade image can
  be directly used to install OpenWrt. Firmware must be upgraded using the
  'force' and 'do not save configuration' command line options (or
  correspondig web interface checkboxes) since the vendor firmware is from
  the pre-DSA era.

Recovery Mode:
 * Press reset button, power up the device, wait for about 10sec.
 * Upload sysupgrade image through the firmware recovery mode web page at
  192.168.1.1.

Signed-off-by: Kim Namu <namu@theseed.io>