ath79: add support for Buffalo WZR-HP-G302H A1A0

ath79: add support for Buffalo WZR-HP-G302H A1A0

Buffalo WZR-HP-G302H is a 2T2R 2.4 GHz 11n router, based on Atheros
AR7242.

It is Japanese market model of WZR-HP-G300NH2, but there are some
diffrences. This commit is based on WZR-HP-G300NH2 in ar71xx.
And, G302H has several hardware versions and hardware is different
dependent on the versions. This commit adds support for "A1A0"
version.

Specification:

- Atheros AR7242
- 64 MB of RAM (DDR2)
- 32 MB of Flash
  - 2x 16 MB SPI-NOR flash
- 2.4 GHz 2T2R wifi
  - Atheros AR9283
- 5x 10/100/1000 Mbps Ethernet
  - Atheros AR8316
- 7x LEDs, 5x keys
  - LED: 1x gpio-leds, 6x ath9k-leds
  - key: 3x buttons, 2x slide switches
- UART header on PCB
  - Vcc, GND, TX, RX from ethernet port side
  - 115200n8

Flash instruction using factory image:

1. Boot WZR-HP-G302H normaly and connect the computer to its LAN port
2. Access to "http://192.168.11.1/" and move to firmware update page
("ファーム更新")
3. Select the OpenWrt factory image and click update ("更新実行")
button to perform firmware update
4. Wait ~200 seconds to complete flashing

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