ath79: add support for TP-Link TL-WR841HP v2

ath79: add support for TP-Link TL-WR841HP v2

Specifications:
- AR9344 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, 4x LAN, Wifi, PWR, WPS, SYSTEM Leds
- Reset/WPS button
- Serial UART at J4 onboard: 3.3v GND RX TX, 1152008N1

MAC addresses as verified by OEM firmware:

vendor   OpenWrt   address
LAN      eth0      label
WAN      eth1      label + 1
WLAN     phy0      label

The label MAC address was found in u-boot 0x1fc00.

Installation:
To install openwrt,
- set the device's SSID to each of the following lines,
  making sure to include the backticks.
- set the ssid and click save between each line.

`echo "httpd -k"> /tmp/s`
`echo "sleep 10">> /tmp/s`
`echo "httpd -r&">> /tmp/s`
`echo "sleep 10">> /tmp/s`
`echo "httpd -k">> /tmp/s`
`echo "sleep 10">> /tmp/s`
`echo "httpd -f">> /tmp/s`
`sh /tmp/s`

- Now, wait 60 sec.
- After the reboot sequence, the router may have fallen back to
  its default IP address with the default credentials (admin:admin).
- Log in to the web interface and go the the firmware upload page.
  Select "openwrt-ath79-generic-tplink_tl-wr841hp-v2-squashfs-factory.bin"
  and you're done : the system now accepts the openwrt.

Forum support topic:
https://forum.openwrt.org/t/support-for-tplink-tl-wr841hp-v2/69445/

Signed-off-by: Saiful Islam <si87868@gmail.com>