iio: add struct declaration for iio types

Add struct for iio type arrays such as IIO_AVAIL_LIST which can be used
instead of int arrays.

Signed-off-by: Gerald Loacker <gerald.loacker@wolfvision.net>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20221201072220.402585-2-gerald.loacker@wolfvision.net


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