ramips: add support for I-O DATA WN-AC1167GR

ramips: add support for I-O DATA WN-AC1167GR

I-O DATA WN-AC1167GR is a 2.4/5 GHz band 11ac router, based on
MediaTek MT7620A.

Specification:

- SoC     : MediaTek MT7620A
- RAM     : DDR2 64 MB
- Flash   : SPI-NOR 8MB
- WLAN    : 2.4/5 GHz, 2T2R
  - 2.4 GHz: MT7620A (SoC)
  - 5 GHz  : MT7612E
- Ethernet: 10/100/1000 Mbps (ext. MT7530)
- LED/key : 4x/3x (2x buttons, 1x slide-switch)
- UART    : through-hole on PCB
  - J2: TX, GND, RX, Vcc from SoC side
  - 115200n8

Flash instruction using factory image:

1. Boot WN-AC1167GR normaly
2. Access to "http://192.168.0.1/" and open firmware update page
("ファームウェア")
3. Select the OpenWrt factory image and click update ("更新") button
to perform firmware update
4. Wait ~150 seconds to complete flashing

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