regulator: fan53555: Constify struct regmap_config and slew_rates array

The regmap_config struct may be const because it is not modified by the
driver and regmap_init() accepts pointer to const. Make const also
slew_rates array.

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