omap: iommu: omapify 'struct iommu' and exposed API

Prepend 'omap_' to OMAP's 'struct iommu' and exposed API, to prevent
namespace pollution and generally to improve readability of the code
that still uses the driver directly.

Update the users as needed as well.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>