regulator: max597x: Align for simple_mfd_i2c driver

Use regmap provided by simple_mfd_i2c driver and remove unused variable.
Identify device variant by checking compatible property in DT.

Signed-off-by: Naresh Solanki <Naresh.Solanki@9elements.com>
Link: https://lore.kernel.org/r/20230216075302.68935-1-Naresh.Solanki@9elements.com


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