gpio: ath79: Convert to immutable irq_chip

Convert the driver to immutable irq-chip with a bit of
intuition.

Cc: Marc Zyngier <[e-mail]>
Acked-by: Marc Zyngier <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>

drivers/gpio/gpio-ath79.c

···
 	u32 mask = BIT(irqd_to_hwirq(data));
 	unsigned long flags;
 
+	gpiochip_enable_irq(&ctrl->gc, irqd_to_hwirq(data));
 	raw_spin_lock_irqsave(&ctrl->lock, flags);
 	ath79_gpio_update_bits(ctrl, AR71XX_GPIO_REG_INT_MASK, mask, mask);
 	raw_spin_unlock_irqrestore(&ctrl->lock, flags);
···
 	raw_spin_lock_irqsave(&ctrl->lock, flags);
 	ath79_gpio_update_bits(ctrl, AR71XX_GPIO_REG_INT_MASK, mask, 0);
 	raw_spin_unlock_irqrestore(&ctrl->lock, flags);
+	gpiochip_disable_irq(&ctrl->gc, irqd_to_hwirq(data));
 }
 
 static void ath79_gpio_irq_enable(struct irq_data *data)
···
 	return 0;
 }
 
-static struct irq_chip ath79_gpio_irqchip = {
+static const struct irq_chip ath79_gpio_irqchip = {
 	.name = "gpio-ath79",
 	.irq_enable = ath79_gpio_irq_enable,
 	.irq_disable = ath79_gpio_irq_disable,
 	.irq_mask = ath79_gpio_irq_mask,
 	.irq_unmask = ath79_gpio_irq_unmask,
 	.irq_set_type = ath79_gpio_irq_set_type,
+	.flags = IRQCHIP_IMMUTABLE,
+	GPIOCHIP_IRQ_RESOURCE_HELPERS,
 };
 
 static void ath79_gpio_irq_handler(struct irq_desc *desc)
···
 	/* Optional interrupt setup */
 	if (!np || of_property_read_bool(np, "interrupt-controller")) {
 		girq = &ctrl->gc.irq;
-		girq->chip = &ath79_gpio_irqchip;
+		gpio_irq_chip_set_chip(girq, &ath79_gpio_irqchip);
 		girq->parent_handler = ath79_gpio_irq_handler;
 		girq->num_parents = 1;
 		girq->parents = devm_kcalloc(dev, 1, sizeof(*girq->parents),