ramips: add support for ELECOM WRC-1167GHBK2-S

ramips: add support for ELECOM WRC-1167GHBK2-S

ELECOM WRC-1167GHBK2-S is a 2.4/5 GHz band 11ac router, based on
MediaTek MT7621A.

Specification:

- MT7621A (2-Cores, 4-Threads)
- 128 MB of RAM (DDR3)
- 16 MB of Flash (SPI)
- 2T2R 2.4/5 GHz
  - MediaTek MT7615D
- 5x 10/100/1000 Mbps Ethernet
- 6x LEDs, 2x keys
- UART header on PCB
  - Vcc, GND, TX, RX from ethernet port side
  - baudrate: 57600 bps

Flash instruction using factory image:

1. Rename the factory image to "wrc-1167ghbk2-s_v0.00.bin"
2. Connect the computer to the LAN port of WRC-1167GHBK2-S
3. Connect power cable to WRC-1167GHBK2-S and turn on it
4. Access to "http://192.168.2.1/details.html" and open firmware
update page ("手動更新（アップデート）")
5. Select the factory image and click apply ("適用") button
6. Wait ~150 seconds to complete flashing

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