[brcm63xx] make default version 3.7.6

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@35505 3c298f89-4303-0410-b956-a3cf2f4a3e73
[brcm63xx] remove support for 3.6 kernel

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@35506 3c298f89-4303-0410-b956-a3cf2f4a3e73