staging:iio:cdc:ad7150: Add sampling_frequency support

Device uses a fixed sampling frequency. Let us expose it to userspace.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-10-jic23@kernel.org