imx6: add initial 3.13 support

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39573 3c298f89-4303-0410-b956-a3cf2f4a3e73
imx6: drop 3.12 support

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39574 3c298f89-4303-0410-b956-a3cf2f4a3e73