gpiolib: add a warning on gpiochip->to_irq defined

gpiochip->to_irq method is redefined in gpiochip_add_irqchip.

A lot of gpiod driver's still define ->to_irq method, let's give
a gentle warning that they can no longer rely on it, so they can remove
it on ocassion.

Fixes: e0d89728 ("gpio: Implement tighter IRQ chip integration")
Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

drivers/gpio/gpiolib.c

@@ -1489,6 +1489,9 @@ static int gpiochip_add_irqchip(struct gpio_chip *gc,
 		type = IRQ_TYPE_NONE;
 	}
 
+	if (gc->to_irq)
+		chip_warn(gc, "to_irq is redefined in %s and you shouldn't rely on it\n", __func__);
+
 	gc->to_irq = gpiochip_to_irq;
 	gc->irq.default_type = type;
 	gc->irq.lock_key = lock_key;