mfd: Support configuration of WM831x /IRQ output in CMOS mode

Provide platform data allowing the system to set the /IRQ pin into
CMOS mode rather than the default open drain. The default value of
this platform data reflects the default hardware configuration so
there should be no change to existing users.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>