pinctrl: at91-pio4: use irq_set_handler_locked

Use irq_set_handler_locked() as it avoids a redundant lookup of the
irq descriptor.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>