regmap: Provide basic KUnit coverage for the raw register I/O

Simple tests that cover basic raw I/O, plus basic coverage of cache sync
since the caches generate bulk I/O with raw register maps. This could be
more comprehensive but it is good for testing generic code.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230610-regcache-raw-kunit-v1-2-583112cd28ac@kernel.org


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