iio: add resource managed triggered buffer init helpers

Add resource managed devm_iio_triggered_buffer_setup() and
devm_iio_triggered_buffer_cleanup() to automatically clean up triggered
buffers setup by IIO drivers, thus leading to simplified IIO drivers code.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>