ramips: mt7621: add TP-Link EAP613 v1

ramips: mt7621: add TP-Link EAP613 v1

The TP-Link EAP613 v1 is a ceiling-mount 802.11ax access point. It can
be powered via PoE or a DC barrel connector (12V). Connecting to the
UART requires fine soldering and careful manipulation of any soldered
wires.

Device details:
  * SoC: MT7621AT
  * Flash: 16 MiB SPI NOR
  * RAM: 256 MiB DDR3L
  * Wi-Fi:
    * MT7905DA + MT7975D: 2.4 GHz + 5 GHz (DBDC), 2x2:2
    * Two stamped metal antennas (ANT1, ANT2)
    * One PCB antenna (ANT3)
    * One unpopulated antenna (ANT4)
  * Ethernet:
    * 1× 10/100/1000 Mbps port with PoE
  * LEDs:
    * Array of four blue LEDs with one control line
  * Buttons:
    * Reset
  * Board test points:
    * UART: next to CPU RF-shield and power circuits
    * JTAG: under CPU RF-shield (untested)
  * Watchdog: 3PEAK TPV706 (not implemented)

  Althought three antennas are populated, the MT7905DA does not support
  the additional Rx chain for background DFS detection (or Bluetooth)
  according to commit 6cbcc34f50a3 ("ramips: disable unsupported
  background radar detection").

MAC addresses:
  * LAN: 48:22:54:xx:xx:a2 (device label)
  * WLAN 2.4 GHz: 48:22:54:xx:xx:a2
  * WLAN 5 GHz: 48:22:54:xx:xx:a3

  The radio calibration blob stored in flash also contains valid MAC
  addresses for both radio bands (OUI 00:0c:43).

Factory install:
  1. Enable SSH on the device via web interface
  2. Log in with SSH, and run `cliclientd stopcs`
  3. Upload -factory.bin image via web interface. It may be necessary to
     shorten the filename of the image to e.g. 'factory.bin'.

Recovery:
  1. Open the device by unscrewing four screws from the backside
  2. Carefully remove board from the housing
  3. Connect to UART (3.3V):
    * Find test points labelled "VCC", "GND", "UART_TX", "UART_RX"
    * Solder wires to test points or connect otherwise. Be careful not
      to damage the PCB e.g. by pulling on soldered wires.
    * Open console with 115200n8 settings
  4. Interrupt bootloader and use tftpboot to start an initramfs:
        setenv ipaddr $DEVICE_IP
        setenv serverip $SERVER_IP
        tftpboot 84000000 openwrt-initramfs-kernel.bin
        bootm

  DO NOT use saveenv to store modified u-boot environment variables. The
  environment is saved at flash offset 0x30000, which erases part of the
  (secondary) bootloader.

  The device uses two bootloader stages. The first stage will load the
  second stage from a uImage stored at flash offset 0x10000. In case of
  a damaged second stage, the first stage should allow uploading a new
  image via y-modem (untested).

Signed-off-by: Sander Vanheule <sander@svanheule.net>