regulator: da9121: Constify struct regmap_config

`da9121_1ch_regmap_config` and `da9121_2ch_regmap_config` are not
modified and can be declared as const to move their data to a
read-only section.

The pointer that references those structs has been converted to const
accordingly.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20240704-regulator-const-regmap-v1-1-bce0ddef63ea@gmail.com


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