ath79: add support for TP-Link CPE510-v2/v3

ath79: add support for TP-Link CPE510-v2/v3

TP-Link CPE510-v2/v3 is an outdoor wireless CPE for 5 GHz with
one Ethernet port based on Atheros AR9344

Specifications:
 - Based on the same underlying hardware as the TP-Link CPE510
 - Power, LAN, and 4 green LEDs
 - 1 10/100Mbps Shielded Ethernet Port (Passive PoE in)
 - Built-in 13dBi 2x2 dual-polarized directional MIMO antenna
 - Adjustable transmission power from 0 to 23dBm/200mw

Flashing instructions:
 Flash factory image through stock firmware WEB UI
 or through TFTP
 To get to TFTP recovery just hold reset button while powering on for
 around 4-5 seconds and release.
 Rename factory image to recovery.bin
 Stock TFTP server IP:192.168.0.100
 Stock device TFTP adress:192.168.0.254

Signed-off-by: Andrew Cameron <apcameron@softhome.net>
[whitespace fixes]
Signed-off-by: Petr Štetiar <ynezz@true.cz>