edtlib: add "hash" attribute to nodes

Add a new "hash" attribute to all Devicetree EDT nodes. The hash is
calculated on the full path of the node; this means that its value
remains stable across rebuilds.
The hash is checked for uniqueness among nodes in the same EDT.

This computed token is then added to `devicetree_generated.h` and made
accessible to Zephyr code via a new DT_NODE_HASH(node_id) macro.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>