iommu/omap: Protect omap-iopgtable.h against double inclusion

Protect the omap-pgtable.h header against double inclusion in
source code by using the standard include guard mechanism.

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>