ramips: add support for D-Link DAP-X1860 A1

ramips: add support for D-Link DAP-X1860 A1

The DAP-X1860 is a wall-plug AX1800 repeater.

Specifications:
- MT7621, 256 MiB RAM, 128 MiB SPI NAND
- MT7915 + MT7975 2x2 802.11ax (DBDC)
- Ethernet: 1 port 10/100/1000
- LED RSSI bargraph (2x green, 1x red/orange), status
  and RSSI LEDs are incorrectly populated red/orange
  (should be red/green according to documentation)

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:
- tar -xvf DAP-X1860_RevA_Firmware_101b94.bin
- openssl enc -d -md md5 -aes-256-cbc -in FWImage.st2 \
  -out FWImage.st1 -k MB0dBx62oXJXDvt12lETWQ==
- tar -xvf FWImage.st1
- flash kernel_DAP-X1860.bin via Recovery

Signed-off-by: Sebastian Schaper <openwrt@sebastianschaper.net>