devicetree: add DT_PARENT()

This macro takes a node identifier, and returns the parent node's
identifier.

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

include/devicetree.h

@@ -212,6 +212,26 @@
  */
 #define DT_INST(inst, compat) UTIL_CAT(DT_N_INST, DT_DASH(inst, compat))
 
+/**
+ * @brief Get a node identifier for a parent node
+ *
+ * Example devicetree fragment:
+ *
+ *     parent: parent-node {
+ *             child: child-node {
+ *                     ...
+ *             };
+ *     };
+ *
+ * The following generate equivalent node identifiers:
+ *
+ *     DT_NODELABEL(parent)
+ *     DT_PARENT(DT_NODELABEL(child))
+ *
+ * @param node_id node identifier
+ */
+#define DT_PARENT(node_id) UTIL_CAT(node_id, _PARENT)
+
 /**
  * @brief Get a node identifier for a child node
  *