iommu/omap: eliminate the public omap_find_iommu_device() method

Eliminate the public omap_find_iommu_device() method, and don't
expect clients to provide the omap_iommu handle anymore.

Instead, OMAP's iommu driver now utilizes dev_archdata's private iommu
extension to be able to access the required iommu information.

This way OMAP IOMMU users are now able to use the generic IOMMU API without
having to call any omap-specific binding method.

Update omap3isp appropriately.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Cc: Hiroshi Doyu <hdoyu@nvidia.com>