drivers/of: Rename unflatten_dt_node()

This renames unflatten_dt_node() to unflatten_dt_nodes() as it
populates multiple device nodes from FDT blob. No logical changes
introduced.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>