iio:proximity:sx9310: Add frequency in read_avail

Instead of using IIO_DEV_ATTR_SAMP_FREQ_AVAIL sysfs attribute,
add a _FREQ case in read_avail() to display the frequency table.
No change to |sampling_frequency_available| was observed.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20220101203817.290512-2-gwendal@chromium.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>