iommu/omap: Convert to devm_* interfaces

Use the various devm_ interfaces to simplify the cleanup in
probe and remove functions.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>