The npcmgpio_irqchip structure is only copied into another
structure, so make it const.

The opportunity for this change was found using Coccinelle.
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>

Link: https://lore.kernel.org/r/1577864614-5543-17-git-send-email-Julia.Lawall@inria.frSigned-off-by: Linus Walleij <linus.walleij@linaro.org>