brcm63xx: switch to 4.1

Switch brcm63xx to 4.1 to give it some initial testing, but keep 3.18
at least until CC-final is out.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46386 3c298f89-4303-0410-b956-a3cf2f4a3e73