ramips: add support for D-Link DAP-1620 B1

ramips: add support for D-Link DAP-1620 B1

The DAP-1620 rev B is a wall-plug AC1300 repeater.

Specifications:
- MT7621AT, 256 MiB RAM, 16 MiB SPI NOR
- MT7615DN 2x2 802.11n +2x2 802.11ac (DBDC)
- Ethernet: 1 port 10/100/1000
- Status LEDs (1x red+green)
- LED RSSI bargraph (2x green, 1x red+green)

Installation:
- Keep reset button pressed during plug-in
- Web Recovery Updater is at 192.168.0.50
- Upload factory.bin, confirm flashing
  (seems to work best with Chromium-based browsers)

Revert to OEM firmware:
- tail -c+117 DAP1620B1_FW212B03.bin | \
  openssl aes-256-cbc -d -md md5 -out decrypted.bin \
  -k 905503a4e0c3cd3c1ce062246de427a68962347e
- flash decrypted.bin via D-Link Web Recovery

Signed-off-by: Rani Hod <rani.hod@gmail.com>