ramips-mt7620: add support for Netgear EX6120

[fabianh95]

• Must be flashable from vendor firmware
  • Web interface
  • TFTP
  • nmrpflash
• 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())')
    -> netgear-ex6120
• 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) -> MAC-Address match eth0 and br-wan but not br-client (last digit changed)
  • 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 -> led labeled "router" and led labeled "device" was controllable in openwrt but is off with gluon
    • Should show activity -> led labeled "router" and led labeled "device" was controllable in openwrt but is off with gluon
    • Should map to their respective port (or switch, if only one led present) -> no extra led
    • Should show link state and activity -> no extra led
• Outdoor devices only:
  • Added board name to is_outdoor_device function in package/gluon-core/luasrc/usr/lib/lua/gluon/platform.lua
• Cellular devices only:
  • Added board name to is_cellular_device function in package/gluon-core/luasrc/usr/lib/lua/gluon/platform.lua
  • Added board name with modem setup function setup_ncm_qmi to package/gluon-core/luasrc/lib/gluon/upgrade/250-cellular
• Docs:
  • Added Device to docs/user/supported_devices.rst

[rotanid]

@fabianh95 please fix the linter issue:

targets/ramips-mt7620:
34: Wrong indentation type(spaces instead of tabs)