gpio: max77620: Make regmap_irq_chip const

Make the structure const as it is only passed to the function
devm_regmap_add_irq_chip having the corresponding argument as const.
Done using Coccinelle.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>