mfd: mc13xxx-spi: Constify struct regmap_bus

`regmap_mc13xxx_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://lore.kernel.org/r/20240704-mfd-const-regmap_config-v2-23-0c8785b1331d@gmail.com


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