intel_mid_powerbtn: add power button driver for Medfield platform (#3)

The power button is connected to MSIC on Medfield, we will get two
interrupts from IOAPIC when pressing or releasing the power button.

Signed-off-by: Hong Liu <hong.liu@intel.com>
[Minor fixes as noted by Dmitry]
Signed-off-by: Alan Cox <alan@linux.intel.com>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Matthew Garrett <mjg@redhat.com>