ath79: add support for Ubiquiti airCube ISP

ath79: add support for Ubiquiti airCube ISP

The Ubiquiti Network airCube ISP is a cube shaped 2.4 GHz with internal
2x2 MIMO antennas. It can be supplied via a USB connector or via PoE.
There are for 10/100 Mbps ports (1 * WAN + 3 * LAN). There is an
optional PoE passthrough from the first LAN port to the WAN port.

SoC:       Qualcomm / Atheros QCA9533-BL3A
RAM:       64 MB DDR2
Flash:     16 MB SPI NOR
Ethernet:  4x 10/100 Mbps (1 WAN + 3 LAN)
LEDS:      1x via a SPI controller (not yet supported)
Buttons:   1x Reset
Serial:    1x (only RX and TX); 115200 baud, 8N1

Missing points:
- LED not yet supported
- Factory upgrade via web IF or TFTP recovery not yet supported
  (Needs RSA signed images, for details see PR#1958)

The serial port is on a four pin connextor labeled J1 and located
between Ethernet and USB connector. The pinout is:
1. 3V3 (out)
2. Rx (in)
3. Tx (out)
4. GND

Upgrading via serial port / U-Boot:
- Connect the serial port via a level converter
- Power the system and stop U-Boot with pressing any key when `Hit any
  key to stop autoboot` is displayed. Note: Pressing space multiple
  times untill U-Boot reaches that location works well.
- Connect a PC with the IP 192.168.1.100 (or some other in that net)
  running a TFTP-Server to one of the LAN ports. Copy the sysupgrade
  image to the server.
- Set the U-Boot server IP with
    setenv serverip 192.168.1.100
- Load the flash image to RAM with
    tftpboot 0x81000000 sysupgrade.bin
- Erase the flash with
    erase 0x9f050000 0x9ffaffff
- Write the new flash content with
    cp 0x81000000 0x9f050000 ${filesize}
- Reset the device with
    reset

Signed-off-by: Christian Mauderer <oss@c-mauderer.de>
[removed full stop in subject and added lockdown note to commit message]
Signed-off-by: Petr Štetiar <ynezz@true.cz>