dtlib: add DT.move_node()

This helper lets you place a node (really the entire subtree rooted at
that node) elsewhere in the devicetree. This will be useful when
adding system devicetree support, when we'll want to be able to, for
example, move the CPU cluster node selected by the current execution
domain to /cpus.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>