input: stpmic1: Add STPMIC1 onkey driver

The STPMIC1 pmic is able to manage an onkey button. This driver exposes
the STPMIC1 onkey as an input device. It can also be configured to
shut-down the power supplies on a long key-press with an adjustable
duration.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>