ramips: add support for YunCore AX820/HWAP-AX820

ramips: add support for YunCore AX820/HWAP-AX820

There are two versions which are identical apart from the enclosure:
  YunCore AX820: indoor ceiling mount AP with integrated antennas
  YunCore HWAP-AX820: outdoor enclosure with external (N) connectors

Hardware specs:
  SoC: MediaTek MT7621DAT
  Flash: 16 MiB SPI NOR
  RAM: 128MiB (DDR3, integrated)
  WiFi: MT7905DAN+MT7975DN 2.4/5GHz 2T2R 802.11ax
  Ethernet: 10/100/1000 Mbps x2 (WAN/PoE+LAN)
  LED: Status (green)
  Button: Reset
  Power: 802.11af/at PoE; DC 12V,1A
  Antennas: AX820(indoor): 4dBi internal; HWAP-AX820(outdoor): external

Flash instructions:
  The "OpenWRT support" version of the AX820 comes with a LEDE-based
  firmware with proprietary MTK drivers and a luci webinterface and
  ssh accessible under 192.168.1.1 on LAN; user root, no password.
  The sysupgrade.bin can be flashed using luci or sysupgrade via ssh,
  you will have to force the upgrade due to a different factory name.
  Remember: Do *not* preserve factory configuration!

MAC addresses as used by OEM firmware:
  use   address   source
  2g    *:0b      Factory 0x0004 (label)
  5g    *         randomly generated
  lan   *:0c      Factory 0xe000
  wan   *:0d      Factory 0xe000 + 1
The wan MAC can also be found in 0xe006 but is not used by OEM dtb.

Signed-off-by: Clemens Hopfer <openwrt@wireloss.net>