ath79: add support for TP-Link TL-WR841HP v3

ath79: add support for TP-Link TL-WR841HP v3

Specifications:
- QCA9533 SoC, 8 MB nor flash, 64 MB DDR2 RAM
- 2x2 9dBi antenna, wifi 2.4Ghz 300Mbps
- 4x Ethernet LAN 10/100, 1x Ethernet WAN 10/100
- 1x WAN, LAN, Wifi, PWR, WPS, RE Leds
- Reset, Wifi on/off, WPS, RE buttons
- Serial UART at J4 onboard: 3.3v GND RX TX, 1152008N1

Label MAC addresses based on vendor firmware:
LAN      *:ea    label
WAN      *:eb    label +1
2.4 GHz  *:ea    label
The label MAC address in found in u-boot 0x1fc00

Installation:
Upload openwrt-ath79-generic-tplink_tl-wr841hp-v3-squashfs-factory.bin
from stock firmware webgui.
Maybe we need rename to shorten file name due to stock webgui error.

Revert back to stock firmware instructions:
- set your PC to static IP address 192.168.0.66 netmask 255.255.255.0
- download stock firmware from Tp-link website
- put it in the root directory of tftp server software
- rename it to wr841hpv3_tp_recovery.bin
- power on while pressing Reset button until any Led is lighting up
- wait for the router to reboot. done

Forum support topic:
https://forum.openwrt.org/t/support-for-tp-link-tl-wr841hp-v3-router

Signed-off-by: Andy Lee <congquynh284@yahoo.com>
[rebase and squash]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>