regmap: Fix rbtreee build when not using debugfs

The debugfs functions don't stub themselves out quite so well as might
be desirable so provide functions which do do this stubbing.
Reported-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

drivers/base/regmap/regcache-rbtree.c

@@ -170,6 +170,15 @@ static const struct file_operations rbtree_fops = {
 	.llseek		= seq_lseek,
 	.release	= single_release,
 };
+
+static void rbtree_debugfs_init(struct regmap *map)
+{
+	debugfs_create_file("rbtree", 0400, map->debugfs, map, &rbtree_fops);
+}
+#else
+static void rbtree_debugfs_init(struct regmap *map)
+{
+}
 #endif
 
 static int regcache_rbtree_init(struct regmap *map)
@@ -194,7 +203,7 @@ static int regcache_rbtree_init(struct regmap *map)
 			goto err;
 	}
 
-	debugfs_create_file("rbtree", 0400, map->debugfs, map, &rbtree_fops);
+	rbtree_debugfs_init(map);
 
 	return 0;