lib/device_tree: Add method to get phandle

Add a method to retrieve a node's phandle.
Useful for board specific devicetree manipulations.

Change-Id: I966151ad7e82fc678ab4f56cf9b5868ef39398e0
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/26191
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/include/device_tree.h

@@ -147,6 +147,8 @@ struct device_tree_node *dt_find_next_compat_child(struct device_tree_node *pare
 // Look up a node relative to a parent node, through its property value.
 struct device_tree_node *dt_find_prop_value(struct device_tree_node *parent, const char *name,
 				   void *data, size_t size);
+// Return the phandle
+uint32_t dt_get_phandle(struct device_tree_node *node);
 // Write src into *dest as a 'length'-byte big-endian integer.
 void dt_write_int(u8 *dest, u64 src, size_t length);
 // Add different kinds of properties to a node, or update existing ones.