ath79/nand: add support for Netgear WNDR4300SW

ath79/nand: add support for Netgear WNDR4300SW

This patch adds support for the WNDR4300SW, marketed by California ISP
SureWest (hence the 'SW' suffix). Hardware wise, it's identical to the
WNDR4300 v1.

Specifications:
* SoC: Atheros AR9344
* RAM: 128 MB
* Flash: 128 MB NAND flash
* WiFi: Atheros AR9580 (5 GHz) and AR9344 (2,4 GHz)
* Ethernet: 5x 1000Base-T
* LED: Power, WAN, LAN, WiFi, USB, WPS
* UART: on board, to the right of the RF shield at the top of the board

Installation:

* Flashing through the OEM web interface:
  + Connect your computer to the router with an ethernet cable and browse
    to http://192.168.1.1/
  + Log in with the default credentials are admin:password
  + Browse to Advanced > Administration > Firmware Upgrade in the Netgear
    interface
  + Upload the Openwrt firmware: openwrt-ath79-nand-netgear_wndr4300sw-squashfs-factory.img
  + Proceed with the firmware installation and give the device a few
    minutes to finish and reboot.

* Flashing through TFTP:
  + Configure your wired client with a static IP in the 192.168.1.x range,
    e.g. 192.168.1.10 and netmask 255.255.255.0.
  + Power off the router.
  + Press and hold the RESET button (the factory reset button on the bottom
    of the device, with the red circle around it) and turn the router on
    while keeping the button pressed.
  + The power LED will start flashing orange. You can release the button
    once it switches to flashing green.
  + Transfer the image over TFTP:
    $ tftp 192.168.1.1 -m binary -c put openwrt-ath79-nand-netgear_wndr4300sw-squashfs-factory.img

Signed-off-by: Stijn Segers <foss@volatilesystems.org>