arm: omap: Add omap3_defconfig

Having one combined defconfig that is the superset of the individual
defconfigs for OMAP3 platforms is useful for easily finding build
errors. Not to mention convenient as a base if you want to boot several
platforms with a single kernel image.
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>