pinctrl: lantiq: Implement gpio_chip.to_irq

Some drivers require a way to translate GPIO pins to their IRQ numbers.

This patch adds the .to_irq() gpiolib callback to the pinctrl-xway
driver, which returns an IRQ mapping for a given GPIO pin.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Martin Schiller <mschiller@tdt.de>
[Switched ->dev to ->parent]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>