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/adc/hi8435.c b/drivers/iio/adc/hi8435.c
index c73c6c6..c62bdb0 100644
--- a/drivers/iio/adc/hi8435.c
+++ b/drivers/iio/adc/hi8435.c
@@ -400,7 +400,7 @@
 			iio_push_event(idev,
 				       IIO_UNMOD_EVENT_CODE(IIO_VOLTAGE, i,
 						    IIO_EV_TYPE_THRESH, dir),
-				       iio_get_time_ns());
+				       iio_get_time_ns(idev));
 		}
 	}