ramips: add support for ELECOM WRC-2533GST

ramips: add support for ELECOM WRC-2533GST

ELECOM WRC-2533GST is a 2.4/5 GHz band 11ac rotuer, based on
MediaTek MT7621A.

Specification:

- MT7621A (2-Core, 4-Threads)
- 128 MB of RAM (DDR3)
- 16 MB of Flash (SPI)
- 4T4R 2.4/5 GHz wifi
  - MediaTek MT7615
- 5x 10/100/1000 Mbps Ethernet
- 4x LEDs, 6 keys (2x buttons, 1x slide switch)
- UART header on PCB
  - Vcc, GND, TX, RX from ethernet port side
  - baudrate: 57600 bps

Flash instruction using factory image:

1. Connect the computer to the LAN port of WRC-2533GST
2. Connect power cable to WRC-2533GST and turn on it
3. Access to "https://192.168.2.1/" and open firmware update
page ("ファームウェア更新")
4. Select the OpenWrt factory image and click apply ("適用")
button
5. Wait ~150 seconds to complete flashing

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>