regulator: max77857: Constify struct regmap_config

`max77857_remgap_config` is not modified and can be declared as const to
move its data to a read-only section.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20240704-regulator-const-regmap-v1-2-bce0ddef63ea@gmail.com


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