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>

arch/arm/plat-omap/iommu.c

@@ -104,6 +104,9 @@ static int iommu_enable(struct iommu *obj)
 	if (!obj)
 		return -EINVAL;
 
+	if (!arch_iommu)
+		return -ENODEV;
+
 	clk_enable(obj->clk);
 
 	err = arch_iommu->enable(obj);