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>

target/linux/ramips/image/mt7621.mk

@@ -910,3 +910,12 @@ define Device/zbtlink_zbt-wg3526-32m
   SUPPORTED_DEVICES += ac1200pro zbt-wg3526-32M
 endef
 TARGET_DEVICES += zbtlink_zbt-wg3526-32m
+
+define Device/zio_freezio
+  MTK_SOC := mt7621
+  IMAGE_SIZE := 16064k
+  DEVICE_VENDOR := ZIO
+  DEVICE_MODEL := FREEZIO
+  DEVICE_PACKAGES := kmod-mt7603 kmod-mt76x2 kmod-usb3 kmod-usb-ledtrig-usbport wpad-basic
+endef
+TARGET_DEVICES += zio_freezio