dt-bindings: mfd: Document short/long press duration for BD718X7

These values can be overwritten at probe time.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-By: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>