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>

drivers/staging/iio/iio.h

@@ -30,6 +30,7 @@
 enum iio_chan_type {
 	/* real channel types */
 	IIO_IN,
+	IIO_OUT,
 	IIO_CURRENT,
 	IIO_POWER,
 	IIO_ACCEL,

drivers/staging/iio/industrialio-core.c

@@ -47,6 +47,7 @@ static const char * const iio_chan_type_name_spec_shared[] = {
 	[IIO_TIMESTAMP] = "timestamp",
 	[IIO_ACCEL] = "accel",
 	[IIO_IN] = "in",
+	[IIO_OUT] = "out",
 	[IIO_CURRENT] = "current",
 	[IIO_POWER] = "power",
 	[IIO_IN_DIFF] = "in-in",
@@ -675,7 +676,8 @@ static int iio_device_add_channel_sysfs(struct iio_dev *dev_info,
 	else
 		ret = __iio_add_chan_devattr("raw", NULL, chan,
 					     &iio_read_channel_info,
-					     NULL,
+					     (chan->type == IIO_OUT ?
+					     &iio_write_channel_info : NULL),
 					     0,
 					     0,
 					     &dev_info->dev,