regulator: max77693: Support different register configurations

Add support for different configurations of charger's registers so the
same driver could be used on other devices (e.g. MAX77843).

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>