gpio: guard gpiochip_irqchip_add_domain() with GPIOLIB_IRQCHIP

The current code doesn't check if GPIOLIB_IRQCHIP is enabled, which results in
a compilation error when trying to build gpio-regmap if CONFIG_GPIOLIB_IRQCHIP
isn't enabled.

Fixes: 6a45b0e2 ("gpiolib: Introduce gpiochip_irqchip_add_domain()")
Suggested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Michael Walle <michael@walle.cc>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20210324081923.20379-2-noltari@gmail.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio/driver.h

@@ -624,8 +624,17 @@ void gpiochip_irq_domain_deactivate(struct irq_domain *domain,
 bool gpiochip_irqchip_irq_valid(const struct gpio_chip *gc,
 				unsigned int offset);
 
+#ifdef CONFIG_GPIOLIB_IRQCHIP
 int gpiochip_irqchip_add_domain(struct gpio_chip *gc,
 				struct irq_domain *domain);
+#else
+static inline int gpiochip_irqchip_add_domain(struct gpio_chip *gc,
+					      struct irq_domain *domain)
+{
+	WARN_ON(1);
+	return -EINVAL;
+}
+#endif
 
 int gpiochip_generic_request(struct gpio_chip *gc, unsigned int offset);
 void gpiochip_generic_free(struct gpio_chip *gc, unsigned int offset);