ath79-generic: add support for Sophos AP55

[herrbett]

Device is very similar to Devolo Wifi Pro series of devices.

This is the first Device of the Series, i have tested. Sophos AP55c, AP100 and AP100c will follow soonish.

• Must be flashable from vendor firmware
  • Web interface
  • TFTP
  • Other: Vendor Software, or TFTP+Serial
• 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
  • Device does not have a reset button, however the serial port is directly accessible
• 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)
• Docs:
  • Added Device to docs/user/supported_devices.rst