ARM: omap2plus_defconfig: Enable OMAP IOMMU support

OMAP3 media platform drivers (i.e: omap3isp) needs IOMMU support so
enable it to be able to test the OMAP3 Image Signal Processor (ISP).

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>