ath79: add support for AirTight C-75

ath79: add support for AirTight C-75

AirTight Networks (later renamed to Mojo Networks) C-75 is a dual-band
access point, also sold by WatchGuard under name AP320.

Specification
SoC: Qualcomm Atheros QCA9550
RAM: 128 MiB DDR2
Flash: 2x 16 MiB SPI NOR
WIFI: 2.4 GHz 3T3R integrated
      5 GHz 3T3R QCA9890 oversized Mini PCIe card
Ethernet: 2x 10/100/1000 Mbps QCA8334
          port labeled LAN1 is PoE capable (802.3at)
USB: 1x 2.0
LEDs: 7x which two are GPIO controlled, four switch controlled, one
      controlled by wireless driver
Buttons: 1x GPIO controlled
Serial: RJ-45 port, Cisco pinout
        baud: 115200, parity: none, flow control: none
JTAG: Yes, pins marked J1 on PCB

Installation
1. Prepare TFTP server with OpenWrt initramfs-kernel image.
2. Connect to one of LAN ports.
3. Connect to serial port.
4. Power on the device and when prompted to stop autoboot, hit any key.
5. Adjust "ipaddr" and "serverip" addresses in U-Boot environment, use
   'setenv' to do that, then run following commands:
    tftpboot 0x81000000 <openwrt_initramfs-kernel_image_name>
    bootm 0x81000000
6. Wait about 1 minute for OpenWrt to boot.
7. Transfer OpenWrt sysupgrade image to /tmp directory and flash it
   with:
    sysupgrade -n /tmp/<openwrt_sysupgrade_image_name>
8. After flashing, the access point will reboot to OpenWrt. Wait few
   minutes, until the Power LED stops blinking, then it's ready for
   configuration.

Known issues
Green power LED does not work.

Additional information
The U-Boot fails to initialise ethernet ports correctly when a UART
adapter is attached to UART pins (marked J3 on PCB).

Cc: Vladimir Georgievsky <vladimir.georgievsky@yahoo.com>
Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>