mfd: ab8500: prepare to handle AB8500 GPIO's IRQs correctly

In an upcoming patch, the gpio-ab8500 driver will relinquish all
IRQ handling capability and pass it back into the AB8500 core
driver. This will aid in reducing massive code duplication within
the kernel. Also, most of the functionality is already in the
AB8500 core driver, as the GPIO IRQs are actually sandwiched
between lots of other IRQs which the core driver already handles.

All we're doing here is providing the core driver with knowledge
that each GPIO has two IRQs assigned to it; one for rising and
a separate one for falling.

Cc: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>