ramips: add support for Belkin RT1800

Belkin RT1800 is an 802.11ax (Wi-Fi 6) router, based on MediaTek
MT7621A.

Specifications:
- SoC: MT7621 (880MHz, 2 Cores)
- RAM: 256 MB
- Flash: 128 MB NAND
- Wi-Fi:
  - MT7915D: 2.4/5 GHz (DBDC)
- Ethernet: 5x 1GiE MT7530
- USB: 1x USB 3.0
- UART: J4 (57600 baud)
  - Pinout: [3V3] (TXD) (RXD) (blank) (GND)

Notes:
* This device has a dual-boot partition scheme, but this firmware works
  only on boot partition 1.

Installation:

Upload the generated factory.bin image via the stock web firmware
updater.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

target/linux/ramips/mt7621/base-files/lib/upgrade/platform.sh

@@ -57,6 +57,7 @@ platform_do_upgrade() {
 	beeline,smartbox-flash|\
 	beeline,smartbox-giga|\
 	beeline,smartbox-turbo|\
+	belkin,rt1800|\
 	dlink,dir-1960-a1|\
 	dlink,dir-2640-a1|\
 	dlink,dir-2660-a1|\