ath79/nand: add support for Netgear WNDR4300TN

ath79/nand: add support for Netgear WNDR4300TN

This patch adds support for the WNDR4300TN, marketed by Belgian ISP
Telenet. The hardware is the same as the WNDR4300 v1, without the
fifth ethernet port (WAN) and the USB port. The circuit board has
the traces, but the components are missing.

Specifications:
* SoC: Atheros AR9344
* RAM: 128 MB
* Flash: 128 MB NAND flash
* WiFi: Atheros AR9580 (5 GHz) and AR9344 (2.4 GHz)
* Ethernet: 4x 1000Base-T
* LED: Power, LAN, WiFi 2.4GHz, WiFi 5GHz, 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://192.168.0.51/
  + Log in with the default credentials are admin:password
  + Browse to Advanced > Administration > Firmware Upgrade in the Telenet
    interface
  + Upload the Openwrt firmware: openwrt-ath79-nand-netgear_wndr4300tn-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 gray circle around it, next to the Telenet logo)
    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_wndr4300tn-squashfs-factory.img

Signed-off-by: Davy Hollevoet <github@natox.be>
[use DT label reference for adding LEDs in DTSI files]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>