regmap: Add a test case for write only registers

Validate that attempts to read from write only registers fail and don't
somehow trigger spurious hardware accesses.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230613-regmap-kunit-read-write-v1-2-2db337c52827@kernel.org


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