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/tsl2563.c b/drivers/iio/light/tsl2563.c
index 57b108c..04598ae 100644
--- a/drivers/iio/light/tsl2563.c
+++ b/drivers/iio/light/tsl2563.c
@@ -630,7 +630,7 @@
 					    0,
 					    IIO_EV_TYPE_THRESH,
 					    IIO_EV_DIR_EITHER),
-		       iio_get_time_ns());
+		       iio_get_time_ns(dev_info));
 
 	/* clear the interrupt and push the event */
 	i2c_smbus_write_byte(chip->client, TSL2563_CMD | TSL2563_CLEARINT);