ath79: add TP-Link WR842N v3

This adds support for the TP-Link WR842N v3 which is already supported on ar71xx
target (0b45bec22c).

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>

target/linux/ath79/base-files/etc/board.d/01_leds

@@ -163,7 +163,8 @@ tplink,tl-wr842n-v2)
 	ucidef_set_led_switch "lan4" "LAN4" "tp-link:green:lan4" "switch0" "0x02"
 	;;
 tplink,tl-wr841-v9|\
-tplink,tl-wr841-v11)
+tplink,tl-wr841-v11|\
+tplink,tl-wr842n-v3)
 	ucidef_set_led_netdev "wan" "WAN" "tp-link:green:wan" "eth0"
 	ucidef_set_led_switch "lan1" "LAN1" "tp-link:green:lan1" "switch0" "0x10"
 	ucidef_set_led_switch "lan2" "LAN2" "tp-link:green:lan2" "switch0" "0x08"

target/linux/ath79/base-files/etc/board.d/02_network

@@ -180,6 +180,7 @@ ath79_setup_interfaces()
 	tplink,tl-mr3420-v1|\
 	tplink,tl-wr841-v9|\
 	tplink,tl-wr841-v11|\
+	tplink,tl-wr842n-v3|\
 	ubnt,airrouter)
 		ucidef_set_interface_wan "eth0"
 		ucidef_add_switch "switch0" \