mfd: 88pm886: Constify struct regmap_irq_chip and some other structures

'struct regmap_irq_chip' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers.

While at it, also constify some other structures.

On a x86_64, with allmodconfig, as an example:
Before:
======
text data bss dec hex filename
5032 3304 64 8400 20d0 drivers/mfd/88pm886.o

After:
=====
text data bss dec hex filename
5800 2536 64 8400 20d0 drivers/mfd/88pm886.o

Signed-off-by: Christophe JAILLET <[e-mail]>
Link: https://lore.kernel.org/r/1681629840[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/88pm886.c

···
 	.max_register = PM886_REG_RTC_SPARE6,
 };
 
-static struct regmap_irq pm886_regmap_irqs[] = {
+static const struct regmap_irq pm886_regmap_irqs[] = {
 	REGMAP_IRQ_REG(PM886_IRQ_ONKEY, 0, PM886_INT_ENA1_ONKEY),
 };
 
-static struct regmap_irq_chip pm886_regmap_irq_chip = {
+static const struct regmap_irq_chip pm886_regmap_irq_chip = {
 	.name = "88pm886",
 	.irqs = pm886_regmap_irqs,
 	.num_irqs = ARRAY_SIZE(pm886_regmap_irqs),
···
 	.unmask_base = PM886_REG_INT_ENA_1,
 };
 
-static struct resource pm886_onkey_resources[] = {
+static const struct resource pm886_onkey_resources[] = {
 	DEFINE_RES_IRQ_NAMED(PM886_IRQ_ONKEY, "88pm886-onkey"),
 };
 
-static struct mfd_cell pm886_devs[] = {
+static const struct mfd_cell pm886_devs[] = {
 	MFD_CELL_RES("88pm886-onkey", pm886_onkey_resources),
 	MFD_CELL_NAME("88pm886-regulator"),
 	MFD_CELL_NAME("88pm886-rtc"),