iommu/omap: Move to_iommu definition from omap-iopgtable.h

The to_iommu definition is used only locally to the omap-iommu.c
source file, and it has nothing to do with the page attributes
defined in omap-iopgtable.h. So, move the definition out of
omap-iopgtable.h header file.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>