bus: sunxi-rsb: Constify struct regmap_bus

`regmap_sunxi_rsb` 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/20240705-sunxi-rsb-bus-const-regmap_bus-v1-1-129094960ce9@gmail.com


Signed-off-by: Chen-Yu Tsai <wens@csie.org>