mfd: rohm-bd718x7: Constify struct regmap_irq_chip

`bd718xx_irq_chip` 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: Matti Vaittinen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/rohm-bd718x7.c

···
 	REGMAP_IRQ_REG(BD718XX_INT_STBY_REQ, 0, BD718XX_INT_STBY_REQ_MASK),
 };
 
-static struct regmap_irq_chip bd718xx_irq_chip = {
+static const struct regmap_irq_chip bd718xx_irq_chip = {
 	.name = "bd718xx-irq",
 	.irqs = bd718xx_irqs,
 	.num_irqs = ARRAY_SIZE(bd718xx_irqs),