dt-bindings: input: Document STPMIC1 PMIC onkey

The STPMIC1 PMIC is able to manage an onkey button. It can 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>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>