scripts: dts: add dependency information to edtlib

Device tree nodes have a dependency on their parents, as well as other
nodes.  To ensure driver instances are initialized in the proper we
need to identify these dependencies and construct a total order on
nodes that ensures no node is initialized before something it depends
on.

Add a Graph class that calculates a partial order on nodes, taking
into account the potential for cycles in the dependency graph.

When generating devicetree value headers calculate the dependency
graph and document the order and dependencies in the derived files.
In the future these dependencies may be expressed in binding data.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>