iommu/omap: Remove conditional definition of dev_to_omap_iommu()

The dev_to_omap_iommu() is local to the OMAP IOMMU modules, and
need not be defined conditionally. The CONFIG_IOMMU_API dependency
check was added in the past to fix a compilation issue back when
the header resided in the arch/arm layers, and is no longer
needed.

While at this, fix the header against double inclusion as well.

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