regmap: Specifically test writing 0 as a value to sparse caches

Since 0 can look a lot like a NULL pointer when used in a cache some clever
data structures might potentially introduce bugs specific to handling it.
Add some explicit testing of storing 0 as a value in a sparse cache, at the
minute there are no issues and this will stop any appearing in the future.

Signed-off-by: Mark Brown <broonie@kernel.org>