fdt: Add function to return next compatible subnode

We need to iterate through subnodes of a parent, looking only at
compatible nodes. Add a utility function to do this for us.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>
diff --git a/lib/fdtdec.c b/lib/fdtdec.c
index 4a5ab71..76d3808 100644
--- a/lib/fdtdec.c
+++ b/lib/fdtdec.c
@@ -133,6 +133,21 @@
 	return fdt_node_offset_by_compatible(blob, node, compat_names[id]);
 }
 
+int fdtdec_next_compatible_subnode(const void *blob, int node,
+		enum fdt_compat_id id, int *depthp)
+{
+	do {
+		node = fdt_next_node(blob, node, depthp);
+	} while (*depthp > 1);
+
+	/* If this is a direct subnode, and compatible, return it */
+	if (*depthp == 1 && 0 == fdt_node_check_compatible(
+						blob, node, compat_names[id]))
+		return node;
+
+	return -FDT_ERR_NOTFOUND;
+}
+
 int fdtdec_next_alias(const void *blob, const char *name,
 		enum fdt_compat_id id, int *upto)
 {