mfd: retu: Constify read-only regmap structs

The regmap_bus, regmap_irq and regmap_irq_chip structs are not modified
and can be declared as const to move their data to a read-only section.

The pointer to reference the regmap_irq_chip structs has been converted
to const.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://lore.kernel.org/r/20240704-mfd-const-regmap_config-v2-11-0c8785b1331d@gmail.com


Signed-off-by: Lee Jones <lee@kernel.org>