iio: expose iio_device_set_clock

Some IIO devices may want to override the default (realtime) to another
clock source by default.

It can beneficial when timestamps coming from the hardware or underlying
drivers are already in that format.

It can always be overridden by attribute current_timestamp_clock.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>