dm: core: Add ofnode_get_chosen_prop()

Add a function to read a property from the chosen node, providing access
to its length. Update ofnode_get_chosen_string() to make use of it.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/dm/ofnode.h

@@ -520,6 +520,18 @@ int ofnode_count_phandle_with_args(ofnode node, const char *list_name,
  */
 ofnode ofnode_path(const char *path);
 
+/**
+ * ofnode_read_chosen_prop() - get the value of a chosen property
+ *
+ * This looks for a property within the /chosen node and returns its value
+ *
+ * @propname: Property name to look for
+ * @sizep: Returns size of property, or FDT_ERR_... error code if function
+ *	returns NULL
+ * @return property value if found, else NULL
+ */
+const void *ofnode_read_chosen_prop(const char *propname, int *sizep);
+
 /**
  * ofnode_read_chosen_string() - get the string value of a chosen property
  *