ARM: OMAP: iommu: declare a private iommu binding struct

Declare an omap iommu private struct, which binds an iommu user
to its iommu device. This struct should be placed at the iommu user's
dev_archdata so generic IOMMU API can be used without having to
utilize omap-specific plumbing anymore.

While at it, provide an accessor method to ease the retrieval of the
omap_iommu handle from a user device.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Cc: Hiroshi Doyu <hdoyu@nvidia.com>