ath79: add support for Buffalo BHR-4GRV2

ath79: add support for Buffalo BHR-4GRV2

Buffalo BHR-4GRV2 is a wired router, based on Qualcomm Atheros
QCA9558.
Ported from ar71xx target.

Specification:

- Qualcomm Atheros QCA9558
- 64 MB of RAM
- 16 MB of Flash
- 5x 10/100/1000 Ethernet
  - QCA8337N
- 4x LEDs, 2x keys
- UART header on PCB
  - Vcc, TX, RX, GND from LED side
  - 115200n8

Flash instruction using factory image:

1. Connect the computer to the LAN port of BHR-4GRV2
2. Connect power cable to BHR-4GRV2 and turn on it
3. Access to "http://192.168.12.1/" and open firmware update
page ("ファームウェア更新")
4. Select the OpenWrt factory image and click update ("更新実行")
button
5. Wait ~120 seconds to complete flashing

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