iommu/omap: Register driver before setting IOMMU ops

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>