ath79: add support for ZyXEL NWA1121-NI

ath79: add support for ZyXEL NWA1121-NI

Specifications:
 * AR9342, 16 MiB Flash, 64 MiB RAM, 802.11n 2T2R, 2.4 GHz
 * 1x Gigabit Ethernet (AR8035), 802.3af PoE

Installation:
* OEM Web UI is at 192.168.1.2
  login as `admin` with password `1234`
* Flash factory-AABJ.bin

The string `AABJ` needs to be present within the file name of the uploaded
image to be accepted by the OEM Web-based updater, the factory image is
named accordingly to save the user from the hassle of manual renaming.

TFTP Recovery:
* Open the case, connect to TTL UART port (this is the official method
  described by Zyxel, the reset button is useless during power-on)
* Extract factory image (.tar.bz2), serve `vmlinux_mi124_f1e.lzma.uImage`
  and `mi124_f1e-jffs2` via tftp at 192.168.1.10
* Interrupt uboot countdown, execute commands
  `run lk`
  `run lf`
  to flash the kernel / filesystem accordingly

MAC addresses as verified by OEM firmware:
use   address   source
LAN   *:cc      mib0 0x30 ('eth0mac'), art 0x1002 (label)
2g    *:cd      mib0 0x4b ('wifi0mac')

Signed-off-by: Sebastian Schaper <openwrt@sebastianschaper.net>