ath79: add support for D-Link DIR-505

ath79: add support for D-Link DIR-505

This commit adds support for the D-Link DIR-505, previously supported in
ar71xx.

Hardware
--------
SoC:   Atheros AR9330
FLASH: 8M SPI-NOR
RAM:   64M
WIFI:  1T1R 1SS Atheros AR9330
LED:   Power green, Status red
BTN:   WPS, Reset

Installation
------------
Currently, installation is only possible by sysupgrading from an earlier
OpenWrt version, U-Boot TFTP or a modded U-Boot. I do not have the
original bootloader from D-Link on my device anymore, so i cannot test
the factory image.

Signed-off-by: David Bauer <mail@david-bauer.net>