mfd: wm8994: Emulate level triggered interrupts if required

The interrupt controller on the wm8994 series of devices requires a level
triggered parent. If one is not available but a GPIO is available for the
interrupt then emulate.

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