iommu/omap: Add iommu-group support

Support for IOMMU groups will become mandatory for drivers,
so add it to the omap iommu driver.

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