Move the registration of the OMAP IOMMU platform driver before
setting the IOMMU callbacks on the platform bus. This causes
the IOMMU devices to be probed first before the .add_device()
callback is invoked for all registered devices, and allows
the iommu_group support to be added to the OMAP IOMMU driver.

While at this, also check for the return status from bus_set_iommu.
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>