ath79: add support for I-O DATA WN-AC1600DGR2

ath79: add support for I-O DATA WN-AC1600DGR2

I-O DATA WN-AC1600DGR2 is a 2.4/5 GHz band 11ac router, based on
Qualcomm Atheros QCA9557.

Specification:

- Qualcomm Atheros QCA9557
- 128 MB of RAM
- 16 MB of Flash
- 2.4/5 GHz wifi
  - 2.4 GHz: 2T2R (SoC internal)
  - 5 GHz: 3T3R (QCA9880)
- 5x 10/100/1000 Mbps Ethernet
- 6x LEDs, 6x keys (4x buttons, 1x slide switch)
- UART header on PCB
  - Vcc, GND, TX, RX from ethernet port side
  - 115200n8

Flash instruction using factory image:

1. Connect the computer to the LAN port of WN-AC1600DGR2
2. Connect power cable to WN-AC1600DGR2 and turn on it
3. Access to "http://192.168.0.1/" and open firmware update page
("ファームウェア")
4. Select the OpenWrt factory image and click update ("更新") button
5. Wait ~150 seconds to complete flashing

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