kernel: add missing config symbol to 3.13

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39745 3c298f89-4303-0410-b956-a3cf2f4a3e73
brcm63xx: add preliminary support for 3.13

Add support for 3.13 as a development kernel. Mostly untested, only net
booted. If flashed may brick your router or kill your cat.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39746 3c298f89-4303-0410-b956-a3cf2f4a3e73