omap: iommu: stop exporting local functions

Stop exporting functions that are used only within the iommu
driver itself.

Eventually OMAP's iommu driver should only expose API via the generic
IOMMU framework.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.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>