iommu/omap: Store iommu_dev pointer in arch_data

Instead of finding the matching IOMMU for a device using
string comparision functions, store the pointer to the
iommu_dev in arch_data during the omap_iommu_add_device
callback and reset it during the omap_iommu_remove_device
callback functions.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
[s-anna@ti.com: few minor cleanups]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>