regmap: Provide user selectable option to enable regmap

Since apparently enabling all the KUnit tests shouldn't enable any new
subsystems it is hard to enable the regmap KUnit tests in normal KUnit
testing scenarios that don't enable any drivers.  Add a Kconfig option
to help with this and include it in the KUnit all tests config.

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230712-regmap-kunit-enable-v1-1-13e296bd0204@kernel.org


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