iio: adc: ad7606: Move oversampling options in chip info and rework *_avail attributes

Available oversampling ratios and scales can be shown by calling a
common ad7606_show_avail function which takes as parameters the array
which stores the values, together with the size of the array.

Oversampling options are now defined in chip info
structure and they are loaded at probe.

Has_Oversampling attribute was removed because oversampling_num was added
and it is not needed anymore.

The purpose of this patch is to deal with the scale_avail and
oversampling_avail arrays in a generic way. This makes it easier to add
support for new devices which will work with different scales and
oversampling ratios. It is also an intermediate step for adding support
for ad7616 which has different oversampling sampling ratios available.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>