regmap: Add test that writes to write only registers are prevented

We should have error checking that verifies that writes to write only
registers are suppressed, verify that this happens as it should.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230613-regmap-kunit-read-write-v1-1-2db337c52827@kernel.org


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