Documentation: DT: lis302: update wakeup binding

This updated the documentation of the DT binding to
describe the added wakeup threshold and second wakeup
engine.

It also adds a note, that the axis values may be
negative.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Reviewed-by: Éric Piel <eric.piel@tremplin-utc.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>