[omap]: switch to 3.12, enable support for AM33xx/OMAP3

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38692 3c298f89-4303-0410-b956-a3cf2f4a3e73