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>
diff --git a/drivers/gpio/gpio-grgpio.c b/drivers/gpio/gpio-grgpio.c
index 35a0277..0a8f761 100644
--- a/drivers/gpio/gpio-grgpio.c
+++ b/drivers/gpio/gpio-grgpio.c
@@ -332,7 +332,7 @@
 	spin_unlock_irqrestore(&priv->bgc.lock, flags);
 }
 
-static struct irq_domain_ops grgpio_irq_domain_ops = {
+static const struct irq_domain_ops grgpio_irq_domain_ops = {
 	.map	= grgpio_irq_map,
 	.unmap	= grgpio_irq_unmap,
 };