regmap: Handle sparse caches in the default sync

If there is no cache entry available we will get -ENOENT from the cache
implementation, handle this gracefully and skip rather than treating it as
an error.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230325-regcache-sparse-sync-v1-1-2a890239d061@kernel.org