mfd: Flag WM831x /IRQ as a wake source

The WM831x can generate wake events, some unconditionally, so flag
the primary IRQ as a wake source in order to help the CPU treat the
/IRQ signal appropriately.

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