mfd: sprd-sc27xx-spi: Constify struct regmap_bus

`sprd_pmic_regmap` is not modified and can be declared as const to
move its data to a read-only section.

Signed-off-by: Javier Carrasco <[e-mail]>
Reviewed-by: Baolin Wang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/sprd-sc27xx-spi.c

···
 	return 0;
 }
 
-static struct regmap_bus sprd_pmic_regmap = {
+static const struct regmap_bus sprd_pmic_regmap = {
 	.write = sprd_pmic_spi_write,
 	.read = sprd_pmic_spi_read,
 	.reg_format_endian_default = REGMAP_ENDIAN_NATIVE,