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>
Link: https://patch.msgid.link/20240924-regcache-zero-value-v1-1-8a1224214b52@kernel.org


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