kirkwood: add support for Linksys EA3500

http://wiki.openwrt.org/toh/linksys/ea3500

Signed-off-by: Dan Walters <dan@walters.io>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43230 3c298f89-4303-0410-b956-a3cf2f4a3e73