mfd: 88pm80x: Constify read-only regmap structs

mfd: 88pm80x: Constify read-only regmap structs

`pm800_irq`, `pm805_irq` and `pm805_irq_chip` are not modified and can
be declared as const to move their data to a read-only section.

In order to keep the const modifier for the regmap_irq_chip structures,
the pointer used to reference them must be converted to const as well.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://lore.kernel.org/r/20240704-mfd-const-regmap_config-v2-8-0c8785b1331d@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>