libfdt: Add fdt_next_subnode() to permit easy subnode iteration

Iterating through subnodes with libfdt is a little painful to write as we
need something like this:

for (depth = 0, count = 0,
	offset = fdt_next_node(fdt, parent_offset, &depth);
     (offset >= 0) && (depth > 0);
     offset = fdt_next_node(fdt, offset, &depth)) {
	if (depth == 1) {
		/* code body */
	}
}

Using fdt_next_subnode() we can instead write this, which is shorter and
easier to get right:

for (offset = fdt_first_subnode(fdt, parent_offset);
     offset >= 0;
     offset = fdt_next_subnode(fdt, offset)) {
	/* code body */
}

Also, it doesn't require two levels of indentation for the loop body.

Signed-off-by: Simon Glass <sjg@chromium.org>
(Cherry-picked from dtc commit 4e76ec79)
Acked-by: Gerald Van Baren <vanbaren@cideas.com>

lib/libfdt/fdt.c

@@ -202,6 +202,34 @@ int fdt_next_node(const void *fdt, int offset, int *depth)
 	return offset;
 }
 
+int fdt_first_subnode(const void *fdt, int offset)
+{
+	int depth = 0;
+
+	offset = fdt_next_node(fdt, offset, &depth);
+	if (offset < 0 || depth != 1)
+		return -FDT_ERR_NOTFOUND;
+
+	return offset;
+}
+
+int fdt_next_subnode(const void *fdt, int offset)
+{
+	int depth = 1;
+
+	/*
+	 * With respect to the parent, the depth of the next subnode will be
+	 * the same as the last.
+	 */
+	do {
+		offset = fdt_next_node(fdt, offset, &depth);
+		if (offset < 0 || depth < 1)
+			return -FDT_ERR_NOTFOUND;
+	} while (depth > 1);
+
+	return offset;
+}
+
 const char *_fdt_find_string(const char *strtab, int tabsize, const char *s)
 {
 	int len = strlen(s) + 1;