Input: add DA9055 Onkey driver

This is the ONKEY driver of the Dialog DA9055 PMIC and depends on the
DA9055 MFD core driver.

This driver was functionally tested on SMDK6410 board.

Signed-off-by: David Dajun Chen <dchen@diasemi.com>
Signed-off-by: Ashish Jangam <ashish.jangam@kpitcummins.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>