ramips-mt76x8: add RE200 version 4

[misanthropos]

• Must be flashable from vendor firmware

  • Web interface
  • TFTP

• Must support upgrade mechanism

  • Must have working sysupgrade
    • Must keep/forget configuration (sysupgrade [-n], firstboot)
  • Gluon profile name matches autoupdater image name
    (lua -e 'print(require("platform_info").get_image_name())')

• Reset/WPS/... button must return device into config mode

• Primary MAC address should match address on device label (or packaging)
  (https://gluon.readthedocs.io/en/latest/dev/hardware.html#hardware-support-in-packages)

  • When re-adding a device that was supported by an earlier version of Gluon, a
    factory reset must be performed before checking the primary MAC address, as
    the setting from the old version is not reset otherwise.

• Wired network

  • should support all network ports on the device
  • must have correct port assignment (WAN/LAN)
    • if there are multiple ports but no WAN port:
      • the PoE input should be WAN, all other ports LAN
      • otherwise the first port should be declared as WAN, all other ports LAN

• Wireless network (if applicable)

  • Association with AP must be possible on all radios
  • Association with 802.11s mesh must work on all radios
  • AP+mesh mode must work in parallel on all radios

• LED mapping

  • Power/system LED
    • Lit while the device is on
    • Should display config mode blink sequence
      (https://gluon.readthedocs.io/en/latest/features/configmode.html)
  • Radio LEDs
    • Should map to their respective radio
    • Should show activity
  • Switch port LEDs
    • Should map to their respective port (or switch, if only one led present)
    • Should show link state and activity
  • Added Device to docs/user/supported_devices.rst

[maurerle]

For completeness:
related PR #2347 tried to introduce support as well and did mention some issues.
The issues were fixed in upstream openwrt openwrt/openwrt#11855
And are said to be resolved now (also mentioned in gluon IRC).