regmap: Allow reads from write only registers with the flat cache

The flat cache is intended for devices that need the lowest overhead so
doesn't track any sparseness.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230617-regmap-kunit-read-writeonly-flat-v1-1-efd3ed66dec6@kernel.org


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