ath79: fix support for Buffalo WZR-HP-G450H and split to dts/dtsi

ath79: fix support for Buffalo WZR-HP-G450H and split to dts/dtsi

There are many parts that are incorrect or missing in the current
code for Buffalo WZR-HP-G450H in ath79, so its support is broken.

I fixed that issues and split to dts/dtsi files to add support for
Buffalo BHR-4GRV.

And WZR-450HP has the same hardware as WZR-HP-G450H, so I change the
device name to "WZR-HP-G450H/WZR-450HP".

Specification:

- Atheros AR7242
- 64 MB of RAM
- 32 MB of Flash
  - 2x 16 MB SPI-NOR flash
- 3T3R 2.4 GHz wifi
  - SoC internal
- 5x 10/100/1000 Mbps Ethernet
- 6x LEDs, 5x 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 WZR-HP-G450H (or WZR-450HP) 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 update
4. Wait ~200 seconds to complete flashing

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