Add omap4 into omap3_defconfig. Note that this does not
yet boot on omap4, but boots on omap2 and omap3.

Also note that CONFIG_SMP does not currently work on
uniprocessor ARMs.
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>