ar71xx: add support for TL-WR802N Version 2

This patch adds support for the TP-Link TL-WR802N Version 2.

Specification:
- SoC: Qualcomm Atheros QCA9533 (560 MHz, MIPS 24Kc)
- RAM: 32 MiB
- Storage: 4 MiB of FLash on board
- Wireless: Built into QCA9533 (Honey Bee), PHY modes b/g/n
- Ethernet: 1x100M (port0)

Installation through OEM Web Interface:
- Connect to TL-WR802N by Ethernet or Wi-Fi
- Go to http://192.168.0.1

 (default user is "admin" & password is "admin)
- Go to "System Tools -> Firmware Upgrade"
- Browse for firmware "*.factory-us.bin" or  "*.factory-eu.bin" for eu model
- Click upgrade

Installation through tftp:
Note: T_OUT, T_IN and GND on the board must be connected to USB TTL
      Serial Configuration 115200 8n1

- Boot the TL-WR802N
- When "Autobooting in 1 seconds" appears type "tpl" followed by enter
- Connect to the board Ethernet port
    (IPADDR: 192.168.1.1, ServerIP: 192.168.1.10)
- tftpboot 0x80000000 <Firmware Image Name>
- Record the result of "printenv bootcmd"
- Enter "erase <Result of 'printenv bootcmd'> +0x3c0000"
    (e.g erase 0x9f020000 +0x3c0000)
- Enter "cp.b 0x80000000 <Result of 'printenv bootcmd'> 0x3c0000"
    (e.g cp.b 0x80000000 0x9f020000 0x3c0000)
- Enter "bootm <Result of 'printenv bootcmd'>"
    (e.g bootm 0x9f020000)

Signed-off-by: Thomas Roberts <tom.p.roberts@gmail.com>