ar71xx: add support for dir 505 a1

this patch is based on ...

-> https://forum.openwrt.org/viewtopic.php?pid=179902#p179902

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@37703 3c298f89-4303-0410-b956-a3cf2f4a3e73