of: iommu: Increment DT node refcount in of_iommu_set_ops()

We are saving pointer to iommu DT node in of_iommu_set_ops()
hence we should increment DT node ref count.

Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>