drm/sprd: Constify struct regmap_bus

drm/sprd: Constify struct regmap_bus

`regmap_txt_io` 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>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240925-drm-const-regmap-v1-6-e609d502401b@gmail.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>