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 <javier.carrasco.cruz@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240704-mfd-const-regmap_config-v2-24-0c8785b1331d@gmail.comSigned-off-by: Lee Jones <lee@kernel.org>

drivers/mfd/sprd-sc27xx-spi.c

@@ -135,7 +135,7 @@ static int sprd_pmic_spi_read(void *context,
 	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,