ofw_graph: Add functions for graph bindings

Those functions are helpers to work on graph bindings.
graphs are mostly use with video related devices.
See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/graph.txt?id=4436a3711e3249840e0679e92d3c951bcaf25515

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D19877