powerpc/iommu: Add missing of_node_put in iommu_init_early_dart

The device_node pointer is returned by of_find_compatible_node
with refcount incremented. We should use of_node_put() to avoid
the refcount leak.

Signed-off-by: Peng Wu <wupeng58@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220425081245.21705-1-wupeng58@huawei.com