Input: add support for PowerOn button on the AB8500 MFD

Add the PowerOn (PonKey) button support to detect power on/off events.

Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Sundar R Iyer <sundar.iyer@stericsson.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>