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>

drivers/iio/dac/ad5504.c

@@ -223,7 +223,7 @@ static irqreturn_t ad5504_event_handler(int irq, void *private)
 					    0,
 					    IIO_EV_TYPE_THRESH,
 					    IIO_EV_DIR_RISING),
-		       iio_get_time_ns());
+		       iio_get_time_ns((struct iio_dev *)private));
 
 	return IRQ_HANDLED;
 }