mfd: ab8500: provide a irq_set_type() function

In the AB8500 IRQ mask and unmask functions, we rely on testing for
IRQ_TYPE_EDGE_RISING and IRQ_TYPE_EDGE_FALLING interrupts to
physically mask and unmask the correct interrupt lines. In order
for us to do that, the trigger needs to be set in the associated
flags. However, unless a irq_set_type() function pointer is passed
when registering the IRQ chip, the IRQ subsystem will refuse to do
it. For that reason, we're providing one.

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>