omap: iommu: Gracefully fail iommu_enable if no arch_iommu is registered

In a modular build of the iommu code it's possible that the arch iommu code
isn't loaded when trying to enable the iommu. Instead of blindly following a
null pointer return -NODEV in that case.

Signed-off-by: Martin Hostettler <martin@neutronstar.dyndns.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>