ath79: add TP-Link WR842N v3

ath79: add TP-Link WR842N v3

This adds support for the TP-Link WR842N v3 which is already supported on ar71xx
target (0b45bec22c59cc6e6b2fa71dc9a88386c5587132).

Specification:
* SoC: QCA9533 ver 2 rev 0
* 16 MB Flash (gd25q128)
* 64 MB RAM
* 1 WAN 10/100 MBit/s (blue connector)
* 4 LAN 10/100 MBit/s (AR8229; 4 ports; yellow connectors)
* Atheros AR9531 (2,4GHz, two fixed antennas)
* USB
* Reset / WPS button
* WiFi button (rf kill)
* 8 green leds; 1 red/green led
* serial console (115200 8N1, according to the OpenWrt-wiki some soldering is needed)

Installation:
* flash via vendor WebUI (the filename must not exceed certain length)
* sysupgrade from installed OpenWrt (also ar71xx)

Thanks to Holger Drefs for providing the hardware

Tested-by: @kofec (github)
Signed-off-by: Sven Roederer <devel-sven@geroedel.de>