ramips: add support for ZIO FREEZIO

ramips: add support for ZIO FREEZIO

ZIO FREEZIO is a 2.4/5GHz band AC1200 router, based on MediaTek MT7621A.

Specifications:
- SoC: MT7621AT
- RAM: DDR3 128MB
- Flash: SPI NOR 16MB
- WiFi:
  - 2.4GHz: MT7603EN
  - 5GHz: MT7612EN
- Ethernet: 5x 10/100/1000Mbps
  - Switch: SoC internal
- USB: 1x 3.0
- UART:
  - J4: 3.3V, RX, TX, GND (3.3V is the square pad) / 57600 8N1

Notes:
- FREEZIO has almost the same board as WeVO W2914NS v2.
- Stock firmware is based on OpenWrt BB.

MAC addresses in factory partition:
0x0004: WiFi 2.4GHz (label_mac-8)
0x002e: WAN (label_mac)
0x8004: WiFi 5GHz (label_mac-4)
0xe000: LAN (label_mac+1)

Installation via web interface:
1.  Access web admin page and turn on "OpenWrt UI mode".
2.  Flash sysupgrade image through LuCI, with the "Keep settings" option
    OFF.

Revert to stock firmware:
1.  Perform sysupgrade with stock image.
    Make sure to NOT preserve settings.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>
[rebase, use mt7621_wevo_w2914ns-v2.dtsi]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>