ramips: add support for I-O DATA WNPR2600G

ramips: add support for I-O DATA WNPR2600G

I-O DATA WNPR2600G is a 2.4/5 GHz band 11ac router, based on MediaTek
MT7621A.

Specification:

- SoC           : MediaTek MT7621A
- RAM           : DDR3 128 MiB
- Flash         : SPI-NOR 16 MiB
- WLAN          : 2.4/5 GHz 4T4R
  - 2.4 GHz     : MediaTek MT7615
  - 5 GHz       : MediaTek MT7615
- Ethernet      : 10/100/1000 Mbps
  - Switch      : MT7530 (SoC)
- LED/keys      : 4x/3x (2x buttons, 1x slide-switch)
- UART          : through-hole on PCB
  - J1: Vcc, RX, GND, TX from SoC side
  - 57600n8

Flash instruction using factory image:

1. Boot WNPR2600G 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>