linux-wandboard

regmap: Allow rbtree to cache zero default values

Ensure that when we start up in cache only mode we can store defaults of
zero, otherwise if the hardware is unavailable we won't be able to read.

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