iio: trigger: implement devm_iio_trigger_alloc/devm_iio_triger_free

Add a resource managed devm_iio_trigger_alloc()/devm_iio_triger_free()
to automatically clean up triggers allocated by IIO drivers, thus
leading to simplified IIO drivers code.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Kyunmin Park <kyungmin.park@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>