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>