regmap: Provide debugfs dump of the rbtree cache data

Show the register ranges we have in each rbtree node in debugfs, plus
some statistics on how big each node is and the total number of nodes.
It may also be worth collecting data on the ranges of dirty registers
to see if there's much mileage in trying to coalesce writes on sync.

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