staging: iio: add channel info for sampling frequency

Adding channel info IIO_CHAN_INFO_SAMP_FREQ to select
different sampling frequency per channel wise.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>