iio: light: gp2ap002: Constify struct regmap_bus

`gp2ap002_regmap_bus` 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/20240703-iio-cont-regmap_bus-v1-9-34754f355b65@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>