FDT: Add a helper to get the subnode by given name

Sometimes it needs to check if there is a subnode of given node in FDT
by given name. Introduce this helper to get the subnode if it exists.

CC: Rob Herring <robh@kernel.org>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Rob Herring <robh@kernel.org>
Tested-by: Julien Grall <julien.grall@arm.com>
diff --git a/include/linux/of_fdt.h b/include/linux/of_fdt.h
index 901ec01..26c3302 100644
--- a/include/linux/of_fdt.h
+++ b/include/linux/of_fdt.h
@@ -53,6 +53,8 @@
 extern int of_scan_flat_dt(int (*it)(unsigned long node, const char *uname,
 				     int depth, void *data),
 			   void *data);
+extern int of_get_flat_dt_subnode_by_name(unsigned long node,
+					  const char *uname);
 extern const void *of_get_flat_dt_prop(unsigned long node, const char *name,
 				       int *size);
 extern int of_flat_dt_is_compatible(unsigned long node, const char *name);