ath79: add support for Buffalo WHR-G301N

ath79: add support for Buffalo WHR-G301N

Buffalo WHR-G301N is a 2.4 GHz 11n router, based on Atheros AR7240.
Ported from ar71xx target.

Specification:

- Atheros AR7240
- 32 MB of RAM
- 4 MB of Flash
- 2.4 GHz 2T2R wifi
- 5x 10/100 Mbps Ethernet
- 9x LEDs, 4x keys
  - LED: 8x gpio-leds, 1x ath9k-leds
  - key: 2x buttons, 1x slide switch
- UART header on PCB
  - Vcc, GND, TX, RX from LEDs side
  - 115200n8

Flash instruction using factory image:

1. Connect the computer to the LAN port of WHR-G301N
2. Connect power cable to WHR-G301N and turn on it
3. Access to "http://192.168.11.1/" and open firmware update page
("ファーム更新")
4. Select the OpenWrt factory image and click execute ("実行") button
5. Wait ~150 seconds to complete flashing

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
[fix the SUPPORTED_DEVICES to be compatible with the ar71xx image]
Signed-off-by: Mathias Kresin <dev@kresin.me>