ath79: add support for Qxwlan E1700AC v2

ath79: add support for Qxwlan E1700AC v2

E1700AC v2 based on Qualcomm/Atheros QCA9563 + QCA9880.

Specification:

 - 750/400/250 MHz (CPU/DDR/AHB)
 - 128 MB of RAM (DDR2)
 - 8/16 MB of FLASH (SPI NOR)
 - 3T3R 2.4 GHz
 - 3T3R 5 GHz
 - 2 x 10/1000M Mbps Ethernet (RJ45)
 - 1 x MiniPCI-e
 - 1 x SIM (3G/4G)
 - 1 x USB 2.0 Port
 - 5 x LED , 2 x Button(S8-Reset Buttun), 1 x power input
 - UART (J5) header on PCB (115200 8N1)

Flash instruction:

   1.Using tftp mode with UART connection and original LEDE image
      - Configure PC with static IP 192.168.1.10 and tftp server.
      - Rename "openwrt-ar71xx-generic-xxx-squashfs-sysupgrade.bin"
        to "firmware.bin" and place it in tftp server directory.
      - Connect PC with one of LAN ports, power up the router and press
        key "Enter" to access U-Boot CLI.
      - Use the following commands to update the device to LEDE:
        run lfw
      - After that the device will reboot and boot to LEDE.
      - Wait until all LEDs stops flashing and use the router.

   2.Using httpd mode with Web UI connection and original LEDE image
      - Configure PC with static IP 192.168.1.xxx(2-255) and tftp server.
      - Connect PC with one of LAN ports,press the reset button, power up
        the router and keep button pressed for around 6-7 seconds, until
        leds flashing.
      - Open your browser and enter 192.168.1.1,You will see the upgrade
        interface, select "openwrt-ar71xx-generic-xxx-squashfs-
        sysupgrade.bin" and click the upgrade button.
      - After that the device will reboot and boot to LEDE.
      - Wait until all LEDs stops flashing and use the router.

Signed-off-by: 张鹏 <sd20@qxwlan.com>
[cut out of bigger patch, keep swconfig, whitespace fixes]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>