mfd: rohm-bd71828: Constify read-only regmap structs

The 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.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/20240704-mfd-const-regmap_config-v2-13-0c8785b1331d@gmail.com


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