pinctrl/at91: convert driver to use gpiolib irqchip

This converts the AT91 pin control driver to register its
chained irq handler and irqchip using the helpers in the
gpiolib core.

Signed-off-by: Alexander Stein <alexanders83@web.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>