ar71xx: add wireless bgn led support for dir-825-c1

I checked the GPL code drop from D-Link and tried to get the wireless
and LAN switch LEDs to light up. I found some references in
AthSDK/www/DIR-825_C1/bsp.h as well as
AthSDK/www/DIR-825_C1/rootfs/etc/sysconfig/S2gpio.sh, but in the end I
only got the led for 2.4GHz to work. Anyway, here's the patch.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37963 3c298f89-4303-0410-b956-a3cf2f4a3e73