ARM: OMAP: iommu: fix including iommu.h without IOMMU_API selected

If included without IOMMU_API being selected it will break
compilation:

arch/arm/plat-omap/include/plat/iommu.h:
	In function 'dev_to_omap_iommu':
arch/arm/plat-omap/include/plat/iommu.h:148:
	error: 'struct dev_archdata' has no member named 'iommu'

This will be seen when hwmod includes iommu.h to get the
structure for attributes. Also needed for tidspbridge
incremental migration to use iommu code.

Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Omar Ramirez Luna <omar.luna@linaro.org>
Signed-off-by: Paul Walmsley <paul@pwsan.com>

arch/arm/plat-omap/include/plat/iommu.h

@@ -126,6 +126,7 @@ struct omap_iommu_arch_data {
 	struct omap_iommu *iommu_dev;
 };
 
+#ifdef CONFIG_IOMMU_API
 /**
  * dev_to_omap_iommu() - retrieves an omap iommu object from a user device
  * @dev: iommu client device
@@ -136,6 +137,7 @@ static inline struct omap_iommu *dev_to_omap_iommu(struct device *dev)
 
 	return arch_data->iommu_dev;
 }
+#endif
 
 /* IOMMU errors */
 #define OMAP_IOMMU_ERR_TLB_MISS		(1 << 0)