staging:iio: Add caching of scan_timestamp to the core as well as buffers.

This will be needed when multiple buffer support is added.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>