iio: imu: st_lsm6dsx: add wakeup-source option

This add ways for the SoC to wake from accelerometer wake events.

In the suspend function we skip disabling the sensor if wakeup-source
and events are activated.

Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Reviewed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>