regmap: debugfs: "registers" & "access" for single register maps

When a map covers a single register, max_register is equal
to 0, so the "registers" & "access" files were not created.
Now they will be, as register 0 must be readable for such
map to make sense.

Signed-off-by: Pawel Moll <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

drivers/base/regmap/regmap-debugfs.c

···
 	debugfs_create_file("range", 0400, map->debugfs,
 			    map, &regmap_reg_ranges_fops);
 
-	if (map->max_register) {
+	if (map->max_register || regmap_readable(map, 0)) {
 		debugfs_create_file("registers", 0400, map->debugfs,
 				    map, &regmap_map_fops);
 		debugfs_create_file("access", 0400, map->debugfs,