ath79: add support for Buffalo BHR-4GRV

ath79: add support for Buffalo BHR-4GRV

Buffalo BHR-4GRV is a wired router, based on Atheros AR7242.

Specification:

- Atheros AR7242
- 64 MB of RAM
- 32 MB of Flash
  - 2x 16 MB SPI-NOR flash
- 5x 10/100/1000 Mbps Ethernet
- 3x LEDs, 2x keys
- 1x USB 2.0 Type-A
- UART header on PCB
  - JP1: Vcc, GND, TX, RX from reset button side

Flash instruction using factory image:

1. Boot the BHR-4GRV normaly and connect the computer to its LAN port
2. Access to
"http://192.168.11.1/cgi-bin/cgi?req=frm&frm=py-db/firmup.html"
with user "bufpy" and password "otdpopy"
3. Select the OpenWrt factory image and click "OK" button to perform
firmware upgrade
4. Wait ~200 seconds to complete flashing

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