gpio: ep93xx: drop to_irq binding

As ->to_irq is redefined in gpiochip_add_irqchip, having it defined in
driver is useless, so let's drop it.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>