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>

target/linux/ramips/image/mt7621.mk

@@ -7,7 +7,7 @@ include ./common-tp-link.mk
 
 DEFAULT_SOC := mt7621
 
-DEVICE_VARS += ELECOM_HWNAME LINKSYS_HWNAME
+DEVICE_VARS += ELECOM_HWNAME LINKSYS_HWNAME DLINK_HWID
 
 ifdef CONFIG_LINUX_5_10
   DTS_CPPFLAGS += -DDTS_LEGACY
@@ -554,6 +554,22 @@ define Device/cudy_x6
 endef
 TARGET_DEVICES += cudy_x6
 
+define Device/dlink_dap-1620-b1
+  DEVICE_VENDOR := D-Link
+  DEVICE_MODEL := DAP-1620
+  DEVICE_VARIANT := B1
+  DEVICE_PACKAGES := kmod-mt7615-firmware rssileds
+  DLINK_HWID := MT76XMT7621-RP-PR2475-NA
+  IMAGE_SIZE := 16064k
+  IMAGES += factory.bin
+  IMAGE/factory.bin := $$(sysupgrade_bin) | \
+    check-size 11009992 | pad-to 11009992 | \
+    append-md5sum-ascii-salted ffff | \
+    append-string $$(DLINK_HWID) | \
+    check-size
+endef
+TARGET_DEVICES += dlink_dap-1620-b1
+
 define Device/dlink_dap-x1860-a1
   $(Device/dsa-migration)
   IMAGE_SIZE := 53248k