input: Add support for the WM831x ON pin

The WM831x series of PMICs support control of initial power on
through the ON pin on the device with soft control of the pin
at other times. Represent this to userspace as KEY_POWER.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>