mfd: arizona: Use handle_simple_irq for IRQ dispatch chip

We use a dummy IRQ chip to dispatch interrupts to the two seperate IRQ
domains on the Arizona devices. This is just a simple software IRQ chip
and thus the current handle_edge_irq is unnecessary for its needs.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>