omap iommu: Make CONFIG_OMAP_IOMMU_DEBUG selectable

This CONFIG_OMAP_IOMMU_DEBUG option cannot be selected because it's
not visible on menu. Make this option selectable.

Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>