gpio: Constify irq_domain_ops

The irq_domain_ops are not modified by the driver and the irqdomain core
code accepts pointer to a const data.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-mpc8xxx.c

@@ -329,7 +329,7 @@ static int mpc8xxx_gpio_irq_map(struct irq_domain *h, unsigned int irq,
 	return 0;
 }
 
-static struct irq_domain_ops mpc8xxx_gpio_irq_ops = {
+static const struct irq_domain_ops mpc8xxx_gpio_irq_ops = {
 	.map	= mpc8xxx_gpio_irq_map,
 	.xlate	= irq_domain_xlate_twocell,
 };