Documentation: dt: Add #iommu-cells info to OMAP iommu bindings

The OMAP IOMMU bindings is updated to reflect the required #iommu-cells
property.

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>