mfd: ab8500: Use irqdomain to map interrupts

All subdrivers use the irqdomain to demux AB8500 IRQs but here
in the hierarchical path we find a leftover instance using the
hard-coded IRQ base. Convert it to use irqdomain with a
oneliner.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>