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>

Documentation/ABI/testing/sysfs-bus-iio

@@ -32,6 +32,13 @@ Description:
 		Description of the physical chip / device for device X.
 		Typically a part number.
 
+What:		/sys/bus/iio/devices/iio:deviceX/timestamp_clock
+KernelVersion:	4.5
+Contact:	linux-iio@vger.kernel.org
+Description:
+		String identifying current posix clock used to timestamp
+		buffered samples and events for device X.
+
 What:		/sys/bus/iio/devices/iio:deviceX/sampling_frequency
 What:		/sys/bus/iio/devices/iio:deviceX/buffer/sampling_frequency
 What:		/sys/bus/iio/devices/triggerX/sampling_frequency