ath79: Add support for Buffalo WZR-HP-G300NH

ath79: Add support for Buffalo WZR-HP-G300NH

This device is a wireless router working on 2.4GHz band based on
Qualcom/Atheros AR9132 rev 2 SoC and is accompanied by Atheros AR9103
wireless chip and Realtek RTL8366RB/S switches. Due to two different
switches being used also two different devices are provided.

  Specification:
  - 400 MHz CPU
  - 64 MB of RAM
  - 32 MB of FLASH (NOR)
  - 3x3:2 2.4 GHz 802.11bgn
  - 5x 10/100/1000 Mbps Ethernet
  - 4x LED, 3x button, On/Off slider, Auto/On/Off slider
  - 1x USB 2.0
  - bare UART header place on PCB

  Flash instruction:
  - NOTE: Pay attention to the switch variant and choose the image to
    flash accordingly. (dmesg / kernel logs can tell it)
  - Methods for flashing
    - Apply factory image in OEM firmware web-gui.
    - Sysupgrade on top of existing OpenWRT image
    - U-Boot TFPT recovery for both stock or OpenWRT images:
      The device U-boot contains a TFTP server that by default has
      an address 192.168.11.1 (MAC 02:AA:BB:CC:DD:1A). During the boot
      there is a time window, during which the device allows an image to
      be uploaded from a client with address 192.168.11.2. The image will
      be written on flash automatically.

      1) Have a computer with static IP address 192.168.11.2 and the
         router device switched off.
      2) Connect the LAN port next to the WAN port in the device and the
         computer using a network switch.
      3) Assign IP 192.168.11.1 the MAC address 02:AA:BB:CC:DD:1A
         arp -s 192.168.11.1 02:AA:BB:CC:DD:1A
      4) Initiate an upload using TFTP image variant
         curl -T <imagename> tftp://192.168.11.1
      5) Switch on the device. The image will be uploaded subsequently.
         You can keep an eye on the diag light on the device, it should
         keep on blinking for a while indicating the writing of the image.

  General notes:
  - In the stock firmware the MAC address is the same among all
    interfaces so it is left here that way too.

  Recovery:
  - TFTP method
  - U-boot serial console

  Differences to ar71xx platform
  - This device is split in two different targets now due to hardware
    being a bit different under the hood. Dynamic solution within the same
    image is left for later time.
  - GPIOs for a sliding On/Off switch, marked 'Movie engine' on the device
    cover, were the wrong way around and were renamed qos_on -> movie_off,
    qos_off -> movie_on. Associated key codes remained the same they were.

  The device tree source code is mostly based on musashino's work

Signed-off-by: Mauri Sandberg <sandberg@mailfence.com>