iio:core: timestamping clock selection support

Adds a new per-device sysfs attribute "current_timestamp_clock" to allow
userspace to select a particular POSIX clock for buffered samples and
events timestamping.

Following clocks, as listed in clock_gettime(2), are supported:
CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_MONOTONIC_RAW,
CLOCK_REALTIME_COARSE, CLOCK_MONOTONIC_COARSE, CLOCK_BOOTTIME and
CLOCK_TAI.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Acked-by: Sanchayan Maity <maitysanchayan@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/light/apds9960.c b/drivers/iio/light/apds9960.c
index 651d57b..a4304ed 100644
--- a/drivers/iio/light/apds9960.c
+++ b/drivers/iio/light/apds9960.c
@@ -807,7 +807,7 @@
 			       IIO_UNMOD_EVENT_CODE(IIO_INTENSITY, 0,
 						    IIO_EV_TYPE_THRESH,
 						    IIO_EV_DIR_EITHER),
-			       iio_get_time_ns());
+			       iio_get_time_ns(indio_dev));
 		regmap_write(data->regmap, APDS9960_REG_CICLEAR, 1);
 	}
 
@@ -816,7 +816,7 @@
 			       IIO_UNMOD_EVENT_CODE(IIO_PROXIMITY, 0,
 						    IIO_EV_TYPE_THRESH,
 						    IIO_EV_DIR_EITHER),
-			       iio_get_time_ns());
+			       iio_get_time_ns(indio_dev));
 		regmap_write(data->regmap, APDS9960_REG_PICLEAR, 1);
 	}