pinctrl: at91: choose appropriate handler for level interrupts

The current implementation handle both edge and level interrupts with the
'handle_simple_irq' handler.

Level interrupts are active as long as the pin stays at the configured
level (low or high). In this case we have to use 'handle_level_irq' which
mask the interrupt until the handle has treated it.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Tested-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>