iio: industrialio-core: Add IIO_OUT type

Add IIO_OUT type for DAC like devices
In case of IIO_OUT make sure the channel device attribute is writable

Ideally we add a flag to iio_chan_spec that tells the core that
the channel device attribute is writable...

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>