iommu/omap: Make use of 'struct iommu_device'

Modify the driver to register individual iommus and
establish links between devices and iommus in sysfs.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
[s-anna@ti.com: fix some cleanup issues during failures]
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>