ath79: migrate TP-Link TL-MR3420v2 to ath79

Specifications:
- SoC: ar9341
- RAM: 32M
- Flash: 4M
- Ethernet: 5x FE ports
- WiFi: ar9341-wmac

Flash instruction:
Upload generated factory firmware on vendor's web interface.

This changes the key assignment compared to ar71xx support of this
device, since of the two keys on the device one is used as combined
Reset/WPS and the second one as WiFi on/off button.
Despite, the reset button required GPIO_ACTIVE_HIGH to work correctly.

Signed-off-by: Lim Guo Wei <limguowei@gmail.com>
[redo commit message]
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>