ath79: add support for Comfast E314N-v2

ath79: add support for Comfast E314N-v2

Taken code from https://patchwork.ozlabs.org/patch/884850/ that was never
pushed by the author, and adapted to ath79.

The Comfast E314N-V2 is a 2.4 GHz 2x2 radio with a built-in directional
antenna and a second Ethernet port - very similar to the Ubiquiti
NanoStation M2. The Ethernet port features a pass-through PoE capability,
enabled or disabled with a slide switch.

Specifications :
- System-On-Chip: Qualcomm/Atheros QCA9531
- CPU/Speed: 650 MHz
- Flash size: 8 MiB
- RAM: 64 MiB
- 2 Ethernet 1Gbp
- 1 reset button
- 1 switch to choose PoE from LAN or Wan. 48Vdc
- Wifi 2.4 Ghz (b/g/n)
- UART inside the box (3.3V, pins marked on the PCB)

Firmware can be flashed on these units by the following method:
1.) Apply power to the unit
2.) Immediately AFTER applying power, hold down the reset button
3.) The WAN, LAN, and wireless lights will flash - wait three seconds
    (three flashes) and then release the button.
4.) After a second, the lights will flutter quickly and the unit will be
    visible at 192.168.1.1. A web page will be available to enable quick
    and simple uploading and flashing of firmware.

During the boot process, these units also look for a tftp server at
192.168.1.10. If one is present, the firmware can be uploaded as a file
called firmware-auto.bin

Signed-off-by: Joan Moreau <jom@grosjo.net>
[wrapped commit message - fix commit title capitalization]
Signed-off-by: David Bauer <mail@david-bauer.net>