pinctrl: at91-pio4: Get rid of legacy call

By just moving the atmel_gpio_to_irq() and calling the internal
function we can get rid of the driver calling back out into the
deprecated external consumer API.

Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>