Add support for YAML encoded output

YAML encoded DT is useful for validation of DTs using binding schemas.

The YAML encoding is an intermediate format used for validation and
is therefore subject to change as needed. The YAML output is dependent
on DTS input with type information preserved.

Signed-off-by: Grant Likely <grant.likely@arm.com>
[robh: make YAML support optional, build fixes, Travis CI test,
 preserve type information in paths and phandles]
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc.h

@@ -299,6 +299,10 @@ struct dt_info *dt_from_blob(const char *fname);
 void dt_to_source(FILE *f, struct dt_info *dti);
 struct dt_info *dt_from_source(const char *f);
 
+/* YAML source */
+
+void dt_to_yaml(FILE *f, struct dt_info *dti);
+
 /* FS trees */
 
 struct dt_info *dt_from_fs(const char *dirname);