iio: adc: at91-sama5d2_adc: add .read_avail() chan_info ops

Add .read_avail() to chan_info ops which will retrieve the available
oversampling ratio.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220803102855.2191070-10-claudiu.beznea@microchip.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>