ath79: add support for I-O DATA WN-AG300DGR

ath79: add support for I-O DATA WN-AG300DGR

I-O DATA WN-AG300DGR is a 2T2R 2.4/5 GHz 11n router, based on Atheros
AR1022.

WN-AG300DGR does not have an LED to indicates power or system status,
I set "router" LED as OpenWrt status LED.

There is no eeprom data for 5 GHz wlan in "art" partition.

Specification:

- Atheros AR1022
- 64 MB of RAM (DDR2)
- 8 MB of Flash (SPI-NOR)
- 2T2R 2.4/5GHz wifi
  - 2.4 GHz: SoC internal
  - 5 GHz: Atheros AR93x2
- 5x 10/100/1000 Mbps Ethernet
- 6x LEDs, 6x keys (4x buttons, 1x slide switch)
- 1x USB 2.0 Type-A
- UART through-hole on PCB
  - Vcc, GND, TX, RX from ethernet port side
  - 115200n8

Flash instruction using factory image:

1. Connect the computer to the LAN port on WN-AG300DGR
2. Connect power cable to WN-AG300DGR and turn it
3. Access to "http://192.168.0.1/" and open firmware update page
("ファームウェア")
4. Select the OpenWrt factory image and click update ("更新") button
5. Wait ~150 seconds to complete flashing

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>