regmap: debugfs: Add a registers `range' file

This file lists the register ranges in the register map.  The condition
to split the range is based on whether the block is readable or not.

Ensure that we lock the `debugfs_off_cache' list whenever we access
and modify the list.  There is a possible race otherwise between the
read() operations of the `registers' file and the `range' file.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/base/regmap/internal.h

@@ -76,6 +76,7 @@ struct regmap {
 	unsigned int debugfs_tot_len;
 
 	struct list_head debugfs_off_cache;
+	struct mutex cache_lock;
 #endif
 
 	unsigned int max_register;