ath79: add support for GL.iNet 6408/6416 (GL.iNet V1)

ath79: add support for GL.iNet 6408/6416 (GL.iNet V1)

This ports the GL.iNet 6408/6416 from ar71xx.

The GL-Connect GL.iNet v1 routers are basically a TP-Link TL-WR710N with
more DRAM/Flash and console/GPIO header in the same small form-factor.

Specifications:
 - SoC: Atheros AR9331
 - CPU: 400 MHz
 - Flash: 8/16 MiB
 - RAM: 64 MiB
 - WiFi: 2.4 GHz b/g/n (SoC)
 - Ethernet: 2x 100M ports (LAN/WAN)
 - USB: 1x 2.0

The difference between 6408 and 6416 is just the flash size. It looks like
only the 16 MiB version has been advertised, while the 6408 is a modified
version. There are also 1-port versions sold by third parties.

Installation:
Install the sysupgrade image via stock firmware GUI or upload it via uboot
(web-based). The device will be available at 192.168.1.1.

Attention: In ar71xx, the same board name is used for both flash versions.
So, please make sure you flash the correct ath79 image when upgrading.

This has been device-tested on a GL.iNet 6416.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>