regmap-irq: handle const struct regmap_irq_sub_irq_map

The struct instances supplied by the drivers are never modified.
Handle them as const in the regmap core allowing the drivers to put them
into .rodata.

Also add a new entry to const_structs.checkpatch to make sure future
instances of this struct already enter the tree as const.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://patch.msgid.link/20240706-regmap-const-structs-v1-2-d08c776da787@weissschuh.net


Signed-off-by: Mark Brown <broonie@kernel.org>