drivers/of: Specify parent node in of_fdt_unflatten_tree()

This adds one more argument to of_fdt_unflatten_tree() to specify
the parent node of the FDT blob that is going to be unflattened.
In the result, the function can be used to unflatten FDT blob that
represents device sub-tree in PowerNV PCI hotplug driver.

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