ramips: add support for Belkin RT1800

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>