regmap: Add test to make sure we don't sync to read only registers

Ensure that a read only value in the register cache does not result in a
write during regcache_sync().

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230613-regmap-kunit-read-write-v1-3-2db337c52827@kernel.org


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