ath79: add support for I-O DATA WN-AC1600DGR

ath79: add support for I-O DATA WN-AC1600DGR

I-O DATA WN-AC1600DGR is a 2.4/5 GHz band 11ac router, based on
Qualcomm Atheros QCA9557.

Specification:

- SoC:      Qualcomm Atheros QCA9557
- RAM:      128 MB
- Flash:    16 MB
- WLAN:     2.4/5 GHz
  - 2.4 GHz: 2T2R (SoC internal)
  - 5 GHz:   3T3R (QCA9880)
- Ethernet: 5x 10/100/1000 Mbps
  - Switch: QCA8337N
- LED/key:  6x/6x(4x buttons, 1x slide switch)
- UART:     through-hole 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-AC1600DGR
2. Connect power cable to WN-AC1600DGR 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

Alternative flash instruction using initramfs image:

1. Prepare a computer and TFTP server software with the IP address
"192.168.99.8" and renamed OpenWrt initramfs image
"uImageWN-AC1600DGR"
2. Connect between WN-AC1600DGR and the computer with UART
3. Connect power cable to WN-AC1600DGR, press "4" on the serial
console and enter the U-Boot console
4. execute "tftpboot" command on the console and download initramfs
image from the TFTP server
5. execute "bootm" command and boot OpenWrt
6. On initramfs image, download the sysupgrade image to the device
and perform sysupgrade with it
7. Wait ~150 seconds to complete flashing

This commit also removes unnecessary "qca,no-eeprom" property from
the ath10k wifi node.

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