ath79: add support for Qxwlan E750A v4

ath79: add support for Qxwlan E750A v4

Qxwlan E750A v4 is based on Qualcomm QCA9344.

Specification:

 - 560/450/225 MHz (CPU/DDR/AHB)
 - 128 MB of RAM (DDR2)
 - 8/16 MB of FLASH (SPI NOR)
 - 2T2R 5G GHz (AR9344)
 - 2x 10/100 Mbps Ethernet (one port with PoE support)
 - 1x miniPCIe slot (USB 2.0 bus only)
 - 7x LED (6 driven by GPIO)
 - 1x button (reset)
 - 1x DC jack for main power input (9-48 V)
 - UART (J23) and LEDs (J2) headers on PCB

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: Peng Zhang <sd20@qxwlan.com>
[cut out of bigger patch, alter use of DEVICE_VARIANT, merge case
in 01_leds, use lower case for v4]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>