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: NNikita Shubin <nikita.shubin@maquefel.me>
Signed-off-by: NBartosz Golaszewski <bgolaszewski@baylibre.com>