ar71xx: add support for TP-Link TL-WR810N v2

ar71xx: add support for TP-Link TL-WR810N v2

TP-Link TL-WR810N v2 is a compact Wi-Fi router with two Ethernet ports.

Specification:

- QCA9533 (650 MHz)
- 32 MB RAM
- 8 MB Flash
- 300 Mbps Wi-Fi 2.4 GHz
- 2x 10/100 Mbps Ethernet (eth0, eth1)
- UART on testpads on PCB
- 1x LED (GPIO-controlled)
- 1x reset/WPS button
- 1x slide-switch with 2 GPIO inputs

Flash instructions:

- Flash as regular firmware update in Web UI
- Default: 192.168.0.254 / admin / admin
- Select *-factory.bin

When holding reset button during power-up, U-Boot requests a file
'wr810nv1_tp_recovery.bin' from TFTP server at 192.168.0.66

Signed-off-by: Paul Wassi <p.wassi@gmx.at>