pinctrl: ti: ti-iodelay: Constify struct ti_iodelay_reg_data

'struct ti_iodelay_reg_data' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
======
text data bss dec hex filename
17259 1788 16 19063 4a77 drivers/pinctrl/ti/pinctrl-ti-iodelay.o

After:
=====
text data bss dec hex filename
17355 1692 16 19063 4a77 drivers/pinctrl/ti/pinctrl-ti-iodelay.o

Signed-off-by: Christophe JAILLET <[e-mail]>
Link: https://lore.kernel.org/40d6e67ab4[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

drivers/pinctrl/ti/pinctrl-ti-iodelay.c

···
 	.max_register = 0xd1c,
 };
 
-static struct ti_iodelay_reg_data dra7_iodelay_data = {
+static const struct ti_iodelay_reg_data dra7_iodelay_data = {
 	.signature_mask = 0x0003f000,
 	.signature_value = 0x29,
 	.lock_mask = 0x00000400,