regmap: rbtree Expose total memory consumption in the rbtree debugfs entry

Provide a feel of how much overhead the rbtree cache adds to
the game.

[Slightly reworded output in debugfs -- broonie]

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