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>

target/linux/ramips/image/mt7621.mk

@@ -478,6 +478,27 @@ define Device/cudy_x6
 endef
 TARGET_DEVICES += cudy_x6
 
+define Device/dlink_dap-x1860-a1
+  $(Device/dsa-migration)
+  IMAGE_SIZE := 53248k
+  DEVICE_VENDOR := D-Link
+  DEVICE_MODEL := DAP-X1860
+  DEVICE_VARIANT := A1
+  UBINIZE_OPTS := -E 5
+  BLOCKSIZE := 128k
+  PAGESIZE := 2048
+  KERNEL_SIZE := 8192k
+  KERNEL_LOADADDR := 0x82000000
+  KERNEL := kernel-bin | relocate-kernel 0x80001000 | lzma | \
+	fit lzma $$(KDIR)/image-$$(firstword $$(DEVICE_DTS)).dtb
+  IMAGES += factory.bin
+  IMAGE/sysupgrade.bin := sysupgrade-tar | append-metadata
+  IMAGE/factory.bin := append-kernel | pad-to $$(KERNEL_SIZE) | append-ubi | \
+	check-size | elx-header 011b0060 8844A2D168B45A2D
+  DEVICE_PACKAGES := kmod-mt7915-firmware rssileds
+endef
+TARGET_DEVICES += dlink_dap-x1860-a1
+
 define Device/dlink_dir-8xx-a1
   $(Device/dsa-migration)
   IMAGE_SIZE := 16000k