kirkwood: kernel: add missing config option

Buildbot found missing config option for target kirkwood.

Signed-off-by: Jiri Slachta <slachta@cesnet.cz>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38900 3c298f89-4303-0410-b956-a3cf2f4a3e73