iio: mpl3115: add threshold events support

iio: mpl3115: add threshold events support

Add support for pressure and temperature rising threshold events. For
both channels *_en and *_value (in raw units) attributes are exposed.

Since in write_event_config() the ctrl_reg1.active and ctrl_reg4
are modified, accessing the data->ctrl_reg{1,4} in set_trigger_state()
and write_event_config() needs to be now guarded by data->lock.
Otherwise, it would be possible that 2 concurrent threads executing
these functions would access the data->ctrl_reg{1,4} at the same time
and then one would overwrite the other's result.

Signed-off-by: Antoni Pokusinski <apokusinski01@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>