input: atc260x: Add onkey driver for ATC260x PMICs

The Actions Semi ATC260x PMICs are able to manage an onkey button.
This driver exposes the ATC260x onkey as an input device. It can also
be configured to force a system reset on a long key-press with an
adjustable duration.

The currently supported chip variants are ATC2603C and ATC2609A.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>